Organic reaction reagents

H+,H20
reduces an alkene to an alocohol will do hydride shifts

1.Hg(Oac)2,THF/H2O 2. NaBh4
reduces an alkene to an alcohol, with no hydride shifts

1. BH3 2. NaOH H2O2
reduces an alkene to an alcohol at the very end of the molecule

H2,, Pd/C, BaSO4
reduces an alkyne to a cis alkene

Na, NH3
reduces an alkyne to a trans alkene

-NH2
takes a proton off of an alkyne

Mg/THF
takes an alkane and forms a grignard reagent (Mg)

tBuli/THF
takes an alkane and forms a grignard reagent (Li)

H2NNH2, KOH
reduces carbonyls into alkanes (Basic Conditions)

Zn/Hg, HCl
reduces carbonyls into alkanes (Acidic conditions)

Na2Cr2O7, HCl, ∆
substitutes a carboxylic acid for a methly group on a aromatic compound

NaNO2, HCl
turns a primary amine into a diazonium group

AIBN
AIBN is a free radical initiator. Upon heating, it decomposes to give nitrogen gas and 2 free radicals


AlBr3
AlBr3 is a Lewis acid. It is useful for promoting electrophilic aromatic substitution, both in bromination of aromatics and also in the Friedel-Crafts reaction.


AlCl3
Aluminum chloride is a strong Lewis acid. It is used to promote reactions such as chlorination of aromatic compounds, as well as Friedel-Crafts reactions. It can also be used in the Meerwein-Ponndorf-Verley reduction.


BF3
Boron trifluoride is a strong Lewis acid and is commonly used for the formation of thioacetals from ketones (or aldehydes) with thiols. The product is a thioacetal.


BH3
Borane (BH3) is a reagent for the hydroboration of alkenes and alkynes. It is sometimes written as B2H6.


Br2
Bromine will react with alkenes, alkynes, aromatics, enols, and enolates, producing brominated compounds. In the presence of light, bromine will also replace hydrogen atoms in alkanes. Finally, bromine is used to promote the Hoffmann rearrangement of amides to amines.


CH2N2 (Diazomethane)
Diazomethane is used for two main purposes. First, it converts carboxylic acids into methyl esters, and second it is used in the Wolff rearrangement as a means of extending carboxylic acids by one carbon.


Cl2
Chlorine is a very good electrophile. It will react with double and triple bonds, as well as aromatics, enols, and enolates to give chlorinated products. In addition it will substitute Cl for halogens when treated with light (free radical conditions). Finally, it assists with the rearrangement of amines to amines (the Hoffmann rearrangement).


CN (cyanide)
Cyanide ion, commonly written as KCN or NaCN, is a good nucleophile for substitution reactions (such as the SN2). It is also used to form cyanohydrins. Cyanide ion will also catalyze the benzoin condensation.


CrO3
Chromium trioxide is an oxidizing agent for alcohols. Its strength depends on the additives present. When in the presence of pyridine, it will oxidize primary alcohols to aldehydes, as well as secondary alcohols to ketones. When acid is present, primary alcohols and aldehydes are oxidized to carboxylic acids.


CuBr
Copper (I) bromide is used in the Sandmeyer reaction, where an aryl diazoniuim salt is converted to an aryl bromide. Occasionally, it is also used for the preparation of organocuprates.


CuCl
Copper (I) chloride is used for the conversion of aryl diazonium salts to aryl chlorides (the Sandmeyer reaction). It is also occasionally used for the formation of organocuprates.


CuI
Copper (I) Iodide is used to form organocuprates (Gilman reagents) when two equivalents of organolithium reagents are added.


DCC (dicyclohexyl carbodiimide)
Dicyclohexyl carbodiimide (DCC) is a reagent used for the synthesis of amides from amines and carboxylic acids. It is essentially a reagent for dehydration (removes water).


Dess-martin periodinane
Dess-martin periodinane is an oxidizing agent. It will oxidize primary alcohols to aldehydes without going to the carboxylic acid (similar to PSS). It will oxidize secondary alcohols to ketones.


DIBAL (diisobutylaluminum hydride)
Di-isobutyl aluminum hydride (DIBAL) is a strong, bulky reducing agent. It is most useful for the partial reduction of esters to aldehydes. It will also reduce other carbonyl compounds such as amides, aldehydes, ketones, and nitriles.


FCl3
Iron (III) chloride (ferric chloride) is a Lewis acid. It is useful in promoting the chlorination of aromatic compounds with Cl2, as well as in the Friedel-Crafts reaction.


Fe
Iron metal will reduce nitro groups to amines in the presence of an acid such as HCl.


FeBr3
Iron (III) bromide (ferric bromide) is a Lewis acid. It is useful for promoting the bromination of aromatic compounds, as well as in the Friedel-Crafts reaction.


Gilman reagents
Organocuprate reagents (Gilman reagents) are carbon nucleophiles. They will do [1,4] additions to alpha-beta unsaturated ketones, as well as SN2 reactions with certain types of alkyl halides. They can also add to acyl halides to give ketones.


Grignard reagents
Grignard reagents are extremely good nucleophiles and strong bases. They react with many electrophiles containing a C=O (carbonyl) group such as aldehydes, ketones, esters, and carbon dioxide, as well as epoxides.


H2
Hydrogen gas is used for the reduction of alkenes, alkynes, and many other species with multiple bonds, in concert with catalysts such as Pd/C and Pt.


H2CrO4
Chromic acid is a strong acid and an oxidant. It will oxidize secondary alcohols to ketones and primary alcohols to carboxylic acids. It is sometimes generated by using K2Cr2O7 in the presence of a strong acid.


H2O2
Hydrogen peroxide (H2O2) is the preferred oxidant in the hydroboration reaction, resulting in the formation of alcohols. Furthermore, it will oxidize aldehydes to carboxylic acids. This is seen in its use for the oxidative workup in ozonolysis.


H2SO4
Sulfuric acid is a strong acid. It is particularly useful as an acid for elimination reactions, since the conjugate base is a very poor nucleophile. It is used in many other reactions simply as a strong acid.


HBr
Hydrobromic acid is a strong acid. It can add to compounds with multiple bonds such as alkenes and alkynes. It can also react with primary, secondary, and tertiary alcohols fo form alkyl bromides.


HCl
Hydrochloric acid is a strong acid. As a reagent, it can react with multiple bonds in alkenes and alkynes, forming chlorinated compounds. It can also concert alcohols to alkyl chlorides.


Hg(OAc)2
Mercuric acetate is a useful reagent for the oxymercuration of alkenes and alkynes. It makes double bonds more reactive towards nucleophilic attack by nucleophiles such as water and alcohols. The mercury is removed using NaBH4 or H2SO4 in the case of addition to alkynes.


HgSO4
Mercuric sulfate is a Lewis acid. In the presence of aqueous acid (H3O+) it will perform the oxymercuration of alkynes to ketones.


HI
Hydroiodic acid is a strong acid. As a reagent, it can add iodine to compounds with multiple bonds such as alkenes and alkynes. It can also convert alcohols to alkyl halides, and cleave ethers.


HIO4
Periodic acid is an oxidant that is useful for the cleavage of 1,2 diols (vicinal diols) into aldehydes or ketones.


HNO2
Nitrous acid is primarily used to convert aromatic amines to diazonium salts, which can be converted into many different compounds via the Sandmeyer reaction. It can also be made from NaNO2 if a strong acid such as H2SO4 or HCl is added.


HNO3
Nitric acid is a strong acid. It will add NO2 to aromatic compounds (usually in the presence of another acid such as H2SO4) and will also oxidize primary alcohols and aldehydes to carboxylic acids.


I2
Iodine is a good electrophile. It will react with carbon-carbon multiple bonds such as alkenes and alkynes, along with other nucleophiles. It is also used in the iodoform reaction.


KMnO4
Potassium permanganate is a very strong oxidizing agent. It will oxidize primary alcohols and aldehydes to carboxylic acids, secondary alcohols to ketones, form diols from alkenes, and oxidatively cleave carbon-carbon multiple bonds.


KOC(CH3)3
Potassium tert-butoxide is a strong, sterically hindered base. It is the prototypical "bulky base", useful for forming less substituted (Hoffmann) alkenes in elimination reactions.


LDA (lithium diisopropylamide)
Lithium di-isopropylamide (LDA) is a strong, bulky, non-nucleophilic base. It is the reagent of choice for selectively removing a proton from the least hindered carbon next to a ketone. It can also be used to form the Hofmann product in E2 reactions.


Li
Lithium is a metal reducing agent similar to sodium and potassium, although weaker. It will convert alkyl halides to alkyl lithium compounds. It will also form alkoxides from alcohols and reduce aromatic groups (The Birch reduction).


LiAlH[OC(CH3)3]3
LiAlH[OC(CH3)3]3 is a strong but bulky reducing agent. Less reactive than LiAlH4, it will convert acyl halides to aldehydes.


LiAlH4
Lithium aluminum hydride is a very strong reducing agent. It will reduce aldehydes, ketones, esters, and carboxylic acids to alcohols, and amides and nitriles to amines. It will also open epoxides.


Lindlar Catalyst
Lindlar's catalyst is a poisoned palladium metal catalyst that performs partial hydrogenation of alkynes in the presence of hydrogen gas. It always gives the cis-alkene, in contrast to Na/NH3, which gives the trans.


mCPBA (m-chloroperoxybenzoic acid)
meta-chloroperoxybenzoic acid (m-CPBA) is an oxidizing agent. One of its main uses is in the formation of epoxides from alkenes. It will also oxidize ketones to form esters, a reaction known as the Baeyer-Villiger reaction.


Me2S
Dimethyl sulfide is used in the "reductive workup" for ozonolysis, especially when one wants to obtain aldehydes from alkenes. It reduces the ozonide that is formed, leading to formation of dimethyl silfoxide (DMSO)


Mg
Magnesium metal is used for the formation of Grignard reagents from alkyl and alkenyl halides.


MsCl
Methanesulfonyl chloride (mesyl chloride or MsCl) is used to turn alcohols into good leaving groups. It behaves essentially identically to TsCl for this purpose.


Na
Sodium is a strong reducing agent. It will reduce alkynes to trans-alkenes, and form hydrogen gas when added to alcohols, forming alkoxides. It is also used in the Birch reduction of aromatic groups.


NaBH(OAc)3
NaBH(OAc)3 (sodium triacetoxy borohydride) is a reducing agent for the reductive amination of ketones and aldehydes to amines. In this respect, it is identical to sodium cyanoborohydride, NaCNBH3


NaBH4
Sodium borohydride is a reagent for the reduction of ketones and aldehydes, it will also reduce acid halides. It is also used in the oxymercuration reaction to replace mercury with H.


NaCNBH3
Sodium cyanoborohydride is a reducing agent. It is generally used for reductive amination - the reduction of imines to amines. Its common to perform this reaction under slightly acidic conditions.


NaH
Sodium hydride is a very strong base and a poor nucleophile. It is useful for deprotonating alcohols and alkynes, among others. One advantage is that the byproduct is H2, which is a gas.


NaIO4
Sodium periodate is a strong oxidant. It will cleave 1,2 diols (vicinal diols) to give aldehydes and ketones.


NaN3
Sodium azide is a good nucleophile that will participate in SN2 reactions.


NaNH2
Sodium amide is a very strong base, useful for the deprotonation of alkynes and in elimination reactions toward the formation of alkynes from dihalides. It can also be used to generate arynes, which can undergo nucleophilic attack.


NBS (n-bromosuccinimide)
NBS is a source of reactive bromine. It is most often used for allylic bromination and in the formation of halohydrines from alkenes.


NCS (n-chlorosuccinimide)
N-chlorosuccinimide is a source of reactive, electrophilic, chlorine. It is used for the formation of chlorohydrins from alkenes.


NH2NH2
Hydrazine is a good reductant and nucleophile. It is used in the Wolff-Kishner reaction, a way of converting ketones to alkanes. It is also used in the final step of the Gabriel amine synthesis to liberate the free amine.


NH2OH
Hydroxylamine is used to the formation of oximes from aldehydes or ketones. The resulting oximes can be converted into amides using the Beckman rearrangement.


NH3
Ammonia is a base and a nucleophile. It is often used as a solvent in reactions involving Li, Na, and K. It has a fairly low boiling point of -33 celcius.


Ni-B
Nickle boride is a reagent that acts similarly to the Lindlar catalyst, reacting with alkynes to give cis-alkenes.


NIS (n-iodosuccinimide)
N-iodosuccinimide is a source of electrophilic iodine, similar to NBS and NCS. When added to an alkene in the presence of water, it will form iodohydrins.


O3 (ozone)
Ozone will cleave alkenes and alkynes to give carbonyl compounds. This is called oxidative cleavage. The products formed can be dependent on the type of workup used. Reductive workup preserves aldehydes, whereas oxidative workup will oxidize any aldehydes to carboxylic acids.


OH-
Hydroxide ion is a strong base and good nucleophile.


Organolithium reagents
Organolithium reagents are extremely strong bases and good nucleophiles. They react with carbonyl compounds (aldehydes, ketones, esters, etc) and epoxides. Being strong bases, they will also react with groups containing acidic hydrogens.


OsO4
Osmium tetroxide is a reagent for the formation of 1,2-diols (vicinal diols) from alkenes. The selectivity for this reaction is always syn.


P2O5
P2O5 is a dehydration reagent. It is used for conversion of carboxylic acids to anhydrides, and also the formation of nitriles from amides.


Pb(OAc)4
Lead tetraacetate will cleave 1,2-diols (vicinal diols) into aldehydes/ketones, similar to NaIO4 and HIO4.


PBr3
Phosphorus tribromide is a reagent for converting alcohols to alkyl bromides. It will also convert carboxylic acids to acid bromides (acyl bromides)


PCC (pyridinium chlorochromate)
Pyridinium chlorochromate (PCC) is reagent for the oxidation of primary alcohols to aldehydes and secondary alcohols to ketones. It is much milder than reactants such as H2CrO4 and KmnO4, which will oxidize primary alcohols to carboxylic acids.


PCl3
Phosphorus trichloride is a reagents for the conversion of alcohols to alkyl chlorides. It will also convert carboxylic acids to acid chlorides (acyl chlorides)


PCl5
Phosphorus pentachloride will convert alcohols to alkyl chlorides, and carboxylic acids to acid chlorides (acyl chlorides)


Pd-C
Palladium adsorbed on charcoal (carbon) is a heterogeneous catalyst. In the presence of hydrogen gas H2, it will convert alkenes and alkynes to alkanes. The hydrogens are delivered syn.


POCl3
Phosphorus oxychloride (POCl3) is used for the dehydration of alcohols to alkenes. Essentially it converts alcohols to a good leaving group, which is then removed by an added base (often pyridine). It can also be used to convert amides to nitriles.


Potassium phthalimide
Sodium (or potassium) pthalimide is a nitrogen-containing nucleophile used in the Gabriel synthesis. Potassium pthalimide reacts with alkyl halides to form a C-N bond, which is then hydrolyzed by treatment with hydrazine (NH2NH2) to give a primary amine.


PPh3
PPh3 is used for the formation of ylides in the Wittig reaction. It can also be used for reductive workup in the ozonolysis of alkenes.


Pt
Platinum is a "noble metal," is used as a catalyst for the reduction of carbon-carbon multiple bonds in the presence of hydrogen gas.


Pyridine
Pyridine is a mild base. Since it bears no charges it is especially soluble in organic solvents. It is often used in reactions that generate HCl and other strong acids - think of it like a sponge for strong acid.


Ra-Ni (Rayney nickle)
Rayney nickle is a reagent for the reduction (hydrogenation) of double bonds. It sees the most use as a reagent for the replacement of sulfur by hydrogen.


RO-OR (peroxides)
Peroxides are used to initiate free-radical reactions. The oxygen-oxygen is very weak, and will fragment homolytically to generate radicals.


AgNO3 (silver nitrate)
Silver nitrate will react with alkyl halides to form silver halides and the corresponding carbocation.


Ag2O (silver oxide)
Silver oxide is used in the Tollens reaction to oxidize aldehydes to carboxylic acids. This is the basis of a test for the presence of aldehydes, since a mirror of metallic silver will be deposited on the flask. It is also commonly used as a base for the Hofmann elimination.


Sn
In the presence of acid, tin will reduce nitro groups to form amines.


SO3
Sulfur trioxide is a reagent for the sulfonylation of aromatic groups. In the presence of acid, it will lead to the formation of sulfonic acids.


SOBr2
Thionyl bromide is a useful reagent for the formation of alkyl bromides from alcohols, as well as acid bromides (acyl bromides) from carboxylic acids.


SOCl2
Thionyl chloride is used for the formation of alkyl chlorides from alcohols and acid chlorides (acyl chlorides) from carboxylic acids.


TBAF (tetra-n-butylammonium fluoride)
Tetrabutylammonium fluoride (TBAF) is a source of fluoride ion. It is used to cleave silyl ethers, which are common protecting groups for alcohols.


TMSCI
TMSCl is a protecting group for alcohols. When added to alcohols, it forms a silyl ether, which is inert to most reagents except for fluoride ion and acid. Note that the reagent can be written two ways (CH3)3SiCl and TMSCl.


TsCl (toluenesulfonyl chloride)
Tosyl chloride TsCl will convert alcohols to sulfonates, which are excellent leaving groups in elimination and substitution reactions.


TsOH (p-toluenesulfonic acid)
Tosic acid (p-toluenesulfonic acid) is a very strong acid. The conjugate base is a very poor nucleophile, which makes it a good acid for elimination reactions. It is similar to H2SO4


Zn
Zinc is a metal reducing agent. It is useful for the reduction of ozonides, and also in the reduction of nitro groups to amines (in the presence of acid).


Zn-Cu
Zinc-copper couple is reducing agent, used to form carbenes (actually carbenoids) from alkyl dihalides. When these are added to alkenes, they form cyclopropanes.


Zn-Hg
In the presence of acid, zinc amalgam will reduce ketones alkanes, in a process called the Clemmensen reaction.


Magnesium monoperoxypthalate hexahydrate (MMPP)
MMPP forms epoxides

Preparation methods of haloalkanes

Methods of Preparation

There are primarily 4 different types of preparation techniques of Haloalkanes and Haloarenes. They include Preparation of Haloalkanes and Haloarenes from:

• Alcohols
• Hydrocarbons
• Alkenes by addition of hydrogen halides and halogens
• Halogen exchange reaction.

1) Preparation from Alcohols (Haloalkanes)

The most convenient method of preparation of haloalkane is from alcohols. R-OH when reacts with suitable reagents, the reaction results in the formation of R − X. The suitable reagents that help in the reaction are

• Concentrated halogen acids (HX)
• Phosphorus halides (PX5 or PX3)
• Thionyl chloride (SOCl2)

i) The Reaction of Alcohols with Halogen Acid

An organic compound derivative of alcohol reacts with halogen acid (H-X) to form haloalkanes as the major product.

Example-Preparation of Chloroalkanes

Preparation of chloroalkane is an example of the reaction of an alcohol with halogen acid to form haloalkane. In this case, primary alcohol and secondary alcohol react with HCl acid gas to form haloalkane in the presence of anhydrous ZnCl2, which act as a catalyst in this reaction.

Preparation of Bromoalkanes

Hydrogen bromide (HBr) reacts with alcohols to form bromoalkanes. Hydrogen bromide synthesis of the reaction takes place by the reaction of sodium bromide or potassium bromide and H2SO4 (sulphuric acid). In the reaction below, NaBr and H2SO4 react to form HBr which further reacts with alcohol.

Example of Preparation of Bromoethane from Ethanol

The tertiary carbonation which occurs due to tertiary alcohols is more stable than secondary and primary alcohols. Therefore tertiary carbocations offer more stability in comparison to the primary and secondary form of the compound. Hence, tertiary is more reactive than primary and secondary. The order of reactivity is 30 > 20 > 10.

Additionally, the reactivity of haloacids follows the order HI > HBr > HCl > HF. HI is easily and highly reacting haloacid among all of them because the HI bond is weaker than the other three. Therefore, it can be broken easily in comparison to the other halogen bonds to form H+ and I− ions.

However, this preparation method will not be able to synthesize aryl halides/haloarenes. C-OH bond in the phenol structure contains partial double bond character because of the delocalization of lone pair of electrons present on the oxygen atom of the benzene ring. Therefore, the bond formed in the structure cannot be easily broken down by any sort of reaction with haloacids.

Ar – OH + HX → No reaction

ii) The Reaction of Alcohols with Phosphorus halides (PX5 or PX3)

This reaction helps in the formation of Chloroalkanes, bromoalkanes, and iodoalkanes. In this reaction phosphorus halides interchange the functional group of alcohols (–OH) with the corresponding halides. The reaction is as follows:

ROH + PCl5 → RCl + POCL3 + HCl

The above reaction is for the formation of alkyl chloride. Similarly, alkyl bromide or alkyl iodide formation is possible by the reaction of an alcohol with phosphorus tribromide and triiodide. To achieve the reaction, red phosphorus reacts with bromine or iodine by in-situ preparation (during the reaction) of phosphorus tribromide and triiodide.

Note: Fresh preparation of the phosphorus tribromide and phosphorus triiodide is made with red phosphorus and bromine or iodine due to the instability of the compounds. Thus, alcohol reacts with phosphorus trihalides (PX3) to obtain three molecules of alkyl halide. The general overall reaction is

Example: Reaction of ethanol with PCl3 for the formation of chloroethane

iii) The Reaction of Alcohols with Thionyl chloride as Suitable Reagent

This reagent is the most preferred and suitable in between the three reactions of alcohols. Alcohol reacts with Thionyl chloride (SOCl2) to form alkyl chlorides. However, the by-products formed in this reaction are gaseous in nature. Therefore, the by-products can easily escape into the atmosphere, leaving the pure alkyl halide. This method helps in the generation of pure alkyl halide.

2) Preparation of Haloalkanes & Haloarenes from Hydrocarbons

Preparation of Haloalkanes and haloarenes from hydrocarbons is possible by 3 different methods. They are

• Free radical halogenation of haloalkanes
• Electrophilic Substitution Reactions
• Sandmeyer reaction

i) Free Radical Halogenation

Alkyl bromides and alkyl chloride formation are possible by the free radical halogenation reaction. However, radicals are very non-selective in nature. Moreover, radicals are non-specific and highly reactive intermediates that result in the formation of the mixture of products.

For instance bromination or chlorination of free radical results in the formation of a number of haloalkanes. This causes difficulty in the isolation of a single product. Therefore it is not the preferred method for the preparation of haloalkanes. Example- When butane reacts with chlorine in the presence of light as energy, a mixture of product formation takes place.

ii) Electrophilic Substitution Reaction

This method helps in the preparation of haloarenes such as aryl bromides and aryl chlorides. Electrophilic substitution forms the aryl bromides and aryl chlorides by using halogens such as chlorine and bromine in the presence of Lewis acid. However, the reaction requires the following of certain specific condition for the generation of proper electrophile.

For example, the reaction should be conducted in presence of Lewis acid. Additionally, the reaction must be carried out in the dark. The reactions to obtain the electrophiles are

The electrophiles in the above reactions are Cl+ and Br+ and HCl and HBr are the by-products of the reaction. Therefore, the electrophilic substitution reaction for the preparation of aryl bromide and aryl chloride is

Mechanism of Electrophilic Substitution Reaction

In the above reaction, two different isomers of the aryl chlorides are formed. They are Ortho and Para isomer. The π-electron in the benzene ring attacks the Cl+ electrophile to produce an intermediate complex. However, the H+ bond from the intermediate complex moves in order to compensate for the positive charge of the carbon atom.

Thus the reaction forms two different isomers of the product-ortho and para. The melting points of both the isomer differ significantly. And para-isomer has the higher boiling point than ortho-isomer. Therefore, they can be easily separated from each other.

Preparation of aryl chloride and bromide is possible from this reaction. Aryl fluoride formation is not possible due to the high reactivity of the halogen fluorine. Additionally, iodine reaction is also not possible as iodine is reversible in nature. Thus, it requires a strong oxidizing agent such as Conc. HNO3 or HIO4 for the oxidation of HI and converting it to I2. Therefore, driving the reaction in forwarding direction is difficult and requires a strong oxidizing agent

iii) Sandmeyer’s Reaction

Sandmeyer’s Reaction is a two-step method which includes:

• Diazonium salt formation
• Diazonium salt reaction with a cuprous halide (Cu2X2)

Primary aromatic amine reacts with sodium nitrite in the presence of cold mineral acid to form the diazonium salt. In this case,  HNO2is prepared within the reaction by reacting sodium nitrite and HX in the temperature of 273-278K.

Mechanism of the Sandmeyer’s Reaction

In the first step-

NaNO2 + HCl → HNO2 + NaCl

The HNO2 formed in the presence of H+ undergo protonation to form NO+ as the electrophile. The lone pair of the atom from the primary amine will react with the electrophile.to form an intermediate compound which further gives diazonium salt after elimination of H2O. In the second step, the diazonium salt reacts with cuprous halide to form the respective aryl halide

3) Haloalkanes & Haloarenes from Alkenes

Haloalkanes and haloarenes preparation is possible by the addition of halogens (X2) across the double bond of the alkene. It is also possible by the addition of hydrogen halides (HX). In this halogen can be chlorine, bromine or even iodine.

i) Addition of HX

Alkene can be converted to haloalkane by an electrophilic addition reaction. Alkene reacts with HX to form R-X. The order of reactivity of halides with respect to alkenes follows the order HI > HBr > HCl > HF. The general reaction will be

The reaction, in this case, is an example of regioselective reaction. In this type of reaction, we get products in major and minor quantity. Additionally, the reaction follows Markovnikov′s rule of addition for the determination of the major product by the addition of across the double bond of the alkene.

According to Markovnikov’s rule, in an addition reaction of unsymmetrical alkenes, the negative part of the reagent or halogen will attach itself to the carbon that contains less number of hydrogen atoms. For example, prop-1-ene reacts with hydrogen bromide to form 2-bromopropane as a major product.

Peroxide effect (Kharash effect)

There is another possibility where the reaction contradicts Markovnikov’s rule. This effect is known as Peroxide effect/ Kharash effect/ anti-markovnikov’s rule. In this reaction, alkene reacts with HBr in the presence of peroxide. The Br- or the negative part of the reagent will attach itself to the carbon having more number of hydrogen atoms. For example, Prop-1-ene reacts with hydrogen bromide to form 1-bromopropane as a major product in the presence of peroxide.

ii) Addition of Halogens

Similarly, alkenes can also react with halogens (X2). For example, Bromine reacts with an alkene in the presence of carbon tetrachloride (CCl4) to form vic-dibromide.  It is a common test to determine a double bond or an alkene compound. The reaction will release reddish brown colour during the reaction.

4) Haloalkanes and Haloarenes from Halogen exchange reaction

i) Finkelstein Reaction

The last method of preparation of haloalkane and haloarene is halogen exchange reaction. In this reaction, an alkyl chloride or alkyl bromide reacts with sodium iodide in acetone to form alkyl iodides.

The reaction is an equilibrium reaction so there is a possibility of forming other products. The solubility difference of alkyl halides in acetone is used for driving the reaction in the forward direction. We know that sodium iodide is soluble in acetone but NaCl or NaBr are insoluble. Therefore, they precipitate out in the reaction which is easy to remove from the reaction mixture.

ii) Swartz Reaction

In this reaction, alkyl fluorides formation is possible by heating of Alkyl fluorides RBr/RCl. The reaction is carried out in the presence of metallic fluoride such as SbF3, Hg2F2, AgF, CoF2.

Some important points

Close Packed Structures

In order to analyze close packing of constituent particles (ions, molecules or atoms) in a lattice, we get into the assumption that constituent particles (ions, molecules or atoms) are hard spheres of identical shape.



Image 1: Different Types of Layers form closed packed structures

What is Close Packing?

The packing of constituent particles inside lattice in such a way that they occupy maximum available space in the lattice is known as Close Packing.

Close Packing is done in three ways, namely:


One Dimensional Close Packing


Two Dimensional Close Packing


Three Dimensional Close Packing



One Dimensional Close Packing

In one dimension close packing, the spheres are arranged in a row touching each other. In one-dimensional close packing, each sphere is in direct contact with two of its neighbor spheres. The number of nearest spheres to a particle in a lattice is called Coordination Number. Since there are two spheres in direct contact with the one sphere, the coordination number of one-dimensional close packing is 2.

Two Dimensional Close Packing

Two-dimensional close packing is done by stacking rows of hard spheres one above the other. This can be done in two ways:


AAA type


ABA type



AAA Type



Image 2: AA type packing

The packing in which one sphere touches two spheres placed in two different rows one above and one below is called AAA type close packing. The coordination number of AAA type two-dimensional close packing is 4. The AAA type is formed by placing one-dimensional row directly one above the other in both horizontal and vertical directions. It is also called two-dimensional square close packing as the rows of spheres when arranged in vertical and horizontal alignments form a square.

AB Type



Image 3: AB Type Packing

The packing in which the spheres in the second row are located in the depressions of the first row. The ABA type close packing is formed by placing one-dimensional row let’s say B type over the A type close packing and this series continues to form a two dimensional hexagonal. The coordination number of ABA Type packing is 6 as each sphere is in direct contact with 6 other spheres.

In ABA type close-packing we find triangular empty spaces called voids. These are of two types:


Apex of triangle pointing upwards


Apex of triangle pointing downwards
 



Three Dimensional Close Packing

The formation of real lattices and structures take place through three-dimensional close packing. They are formed by stacking two-dimensional layers of spheres one above the other. This can be done by two ways:


Three-dimensional close packing from two dimensional square close packed layers


Three-dimensional close packing from two dimensional hexagonal close packed layers



Three-dimensional close packing from two-dimensional close packed layers

Formation of three-dimensional close packing can be done by placing the second square closed packing exactly above the first one. In this close packing, the spheres are aligned properly in horizontally and vertically. Similarly, by placing more layers one above the other, we can obtain a simple cubic lattice. The unit cell of the simple cubic lattice is called the primitive cubic unit cell.

Three-dimensional close packing from two dimensional hexagonal close packed layers

Three-dimensional close packing can be formed with the help of two-dimensional hexagonal packed layers in two ways:


Stacking the second layer over the first layer


Stacking the third layer over the second layer
 



Stacking the second layer over the first layer

Suppose we take two hexagonal close packed layer ‘A’ and place it over the second layer B ( as both layers have different alignment of spheres) such that spheres of the second layer are placed in the depressions of the first layer. We observe that a tetrahedral void is formed when a sphere of the second layer is right above the void (empty space) of the first layer. Adding further we notice octahedral voids at the points where the triangular voids of the second layer are placed right triangular voids of the first one in such a way that triangular space doesn't overlap. Octahedral voids are bordered by six spheres.



Image 4: Types of voids in three-dimensional closed packing

If there are ‘N’ closed spheres, then:


Number of Octahedral Voids equals to “N”


Number of Tetrahedral Voids equals to “ 2N”



Stacking the third layer over the second layer

There are two possible ways of placing the third layer over the second layer:


By Covering Tetrahedral Voids


By Covering Octahedral Voids



Covering Tetrahedral Voids

In this kind of three-dimensional packing, the spheres of the third layer are aligned right above the spheres of the first layer. If we name the first layer as A and second layer as B, then the pattern will be ABAB… so far and so forth. The structure formed is also called hexagonal close-packed structure also known as HCP.

Covering Octahedral Voids

In this kind of packing the third layer, spheres are not placed with either of the second layer or first layer. If we name the first layer as A, second as B and then the third layer will be C (as it is now a different layer) then the pattern will be ABCABC… The structure formed is also called cubic closed packed (ccp) or face-centred packed cubic structure (fcc). For Example metals like copper and iron crystallize in the structure.

The coordination number in both cases will be 12 as each sphere in the structure is in direct contact with 12 other spheres. The packing is highly efficient and around 74% of the crystal is completely occupied.



Image 5: ABC Type of Close Packing