Halogen Derivatives Chemical Properties

Halogen Derivatives Chemical Properties

Chemical Properties :

Haloalkanes are one of the most organic compounds. These can preparation of a large variety compounds. Some of the reactions given below:

• Nucleophilic substitution reaction.
• Elimination reactions
• Reactions with metals
• Reduction

→ Nucleophilic Substitution Reactions : In haloalkanes. the carbon is bonded to a halogen atom (X = F, Cl, Br, I) which is more electronegative than carbon.

→ Because of high electronegativity of the halogen atom, the carbon halogen (C—X) bond is highly polarised covalent bond.

• Therefore nucleophile attacks on carbon atom and halogen is substituted.
• That is why these reactions are known as nucleophilic substitution reaction (S_N)

There are two types of nucleophiles which participate in substitution reaction of alkyl halides. These are :

Negatively charged species:

Neutral species with at least one lone pair

Nucleophilic reactions are divided into two groups:

Unimolecular NucIeophihc Substitution or S_N1

These reactions proceed in two steps:

→ In first step, heterolytic cleavage of C—X bond, form a carbocation (carbonium ion) (R^⊕) in the form of intermediate and a halide ion X^–. Step I is the slowest step therefore it is rate determining step.

In the first step. only alkyl halide is used, therefore rate of reaction depends only upon the concentration of alkyl halide.

Rate ∝ [R—X]

Therefore this reaction is known as uni-molecular nucleophilic substitution reaction (S_N1).

In second step, the carbocation is attacked by nucleophile to form product.

As the stability of carbocation formed in first step increases, the reaction will easily proceed. Order of stability of carbocation is as follows:

Order of reactivity in towards S_N1 mechanism of halogen atom is same:

→ The intermediate carbocation is planar, therefore attack of the nucleophile may be accomplished from either side resulting in a mixture of products. Therefore, if reactant is optical isomer then product is a racemic mixture.

→ Mechanism of S_N1 reaction between tertiary butyl chloride and aqueus KOH can be explained in following way:

Bimolecular nucleophilic substitution or S_N1

• This type of reaction occurs in one step.
• In this type of reactions, only formation of transition state occurs and no intermediate is formed.
• For the formation of transition state, the attacking nucleophile attacks from the opposite direction i. e., from 180° angle of
• leaving group (X^Θ). This is known as Back side attack or Rear attack.
• In the transition state, attacking nucleophile (Nu^Θ) and leaving group (X^Θ) both are partially attacked with the central carbon atom.

→ The rate of reaction depends upon the concentration of both the alkyl halide and the nucleophile. Therefore these reactions are known as bimolecular nucleophilic substitution reactions.

Rate ∝ [R—X] [Nu^Θ]

→ In these reactions, the configuration of products is opposite to that of reactants therefore in these reaction, inversion in configuration occurs. This is known as “Walden Inversion”.

→ In these reactions, the presence of bulky alkyl group on the carbon atom of C—X bond, produces steric hindrance for the attack of nucleophile. Therefore with the increase in the number of alkyl groups on carbon, the reactivity of R—X decreases. If halogen atom is same, then order of reactivity of alkyl halides towards \(\mathrm{S}_{\mathrm{N}^{2}}\) mechanism is as follows:

→ Therefore, primary alkyl halides react most rapidly by \(\mathrm{S}_{\mathrm{N}^{2}}\) mechanism whereas tertiary alkyl halides react by \(\mathrm{S}_{\mathrm{N}^{1}}\) mechanism. Secondary alkyl halides will react by \(\mathrm{S}_{\mathrm{N}^{1}}\) or \(\mathrm{S}_{\mathrm{N}^{2}}\) mechanism. This depends upon the nature of ncleopile and solvent.

Sorne nucleophilic substitution reactions are as follows:

→ Synthesis of Alkyl Cyanide : Alkyl halides react with alcoholic solution of KCN to give alkyl cyanides.

→ It may be noted that the reaction of haloalkane with alcoholic KCN is very important because the product formed has one more carbon atom than the alkyl halide.

→ Therefore, the reaction is a good method for increasing the length of the carbon chain by one carbon atom. Alkyl cyanides thus obtained, on complete hydrolysis gives acids, on partial hydrolysis gives amides and on reaction form primary amine.

→ Formation of Alkyl Isocyanides : When alkyl halide is treated with AgCN, alkyl isocyanides are obtained.

→ Alkyl isocyanides on hydrolysis form primary amine and on reduction give secondary amines.

Cyanides with KCN and iso-cyanides with AgCN as the product can be explained as follows:

→ KCN is predominantly ionic in nature, therefore, CN^Θ (cyanide) ion is inihe form of nucleophile and the attack mainly occurs through the carbon end of the cyanide ion forming alkylcyanides.

→ On the other hand, AgCN is predominantly covalent and due to the presence of lone pair on nitrogen attack, occurs through N-atom of cyanide group forming alkyl isocyanides.

→ Synthesis of Alcohol : Alkyl halides react with aqueous KOH, moist AgOH or moist Ag₂O to form alcohols.

Order of Reactivity:

3° Alkyl Halide > 2° Alkyl Halide> 1° Alkyl Halide.

→ Therefore tertiary alkyl halides form alcohols only by boiling them with water whereas secondary alkyl halides are also hydrolysed by weak base such as Na₂CO₃ or CaCO₃ and form alcohol.

→ Synthesis of Alkyl Nitrite : When an alkyl halide is treated with sodium or potassium nitrite, alkvl nitrites are formed.

→ Alkali metal nitrite (KNO₂) is ionic compound and the bond between K—O is ionic and therefore, the negative charge on oxygen is attacking site.

Formation of Nitro Alkanes : When alkyl halide is treated with silver nitrite (AgNO₉). nitro-alkanes are formed.

→ Silver nitrite (AgNO₂) is a covalent compound and the bond between Ag—O is covalent. So it does not have negative charge on oxygen. Hence, attack occurs through lone pair of nitrogen atom.

→ Forniation of Ethers : When alkyl halide is treated with dry silver oxide, ether is formed

→ If alkyl halide is treated with sodium alkoxide, then ether is formed and this reaction is called Williamson’s synthesis.

→ This reaction is quite useful for preparing ethers.

→ Formation of Alkane Thiol : On treatment with sodium or potassium hydrogen suiphide, alkyl halides from akane thiols.

→ Formation of Thioethers : When alkyl halide is treated with sodium suiphide (Na₂S) and sodium mercaptide (NaSR), thio ethers are obtained.

→ FormatIon of Eaters : Alkyl halides form esters when heated with silver salts of carboxylic acids.

→ Formation of AmInes : When alkyl halide is heated with alcoholic ammonia solution in a sealed tube, alkylation of ammonia occurs and a mixture of amines is formed. When alkyl halide is in excess, then quaternary ammonium salt is rormed.

Reaction of Methyl chloride with ammonia occurs as follow:

This reaction is called Hofmann anunonolysis reaction.

→ Friedal Craft Reaction : When alkyl halide is treated with henzene in the presence of anhydrous AlCl₃, alkyl benzene is obtained, in this reaction, akylation of benzene occurs.

→ Mechanism : Friedal Craft reaction of benzene is electrophilic subsitution reaction.

→ In this reaction, carbocation is formed as intermediate with the help of AlCl₃ which acts as electrophile in secondary and tertiary alkyl halides.

→ Whereas in primary alkyl halides, formation of

(Alkyl halide aluminium chloride complex) occurs which acts as electrophile.

→ Carbonium ion of aromatic system or positive charge on carbon in aromatic system is known as aremum ion.

→ Elimination Reactions : In these reactions, two atoms or two groups eliminate from a molecule and formation of unsaturated bond occurs. These reactions are known as elimination reactions. When alkyl halides are heated with alcoholic solution of KOH, they undergo 13-elimination of HX molecule resulting in the formation of alkenes.

→ In these reactions, there is elimination of hydrogen atom from 13-carbon. therefore it is known as 13-elimination. Elimination of HX from alkyl halide molecule occurs in this reaction therefore this reaction is also known as ‘dehydrohalogenat ion’.

→ The elimination reaction occurs by abstraction of proton from a carbon atom next to the carbon bearing halogen atom (called 13-hydrogen) and halide ion is also lost resulting a new it-bond as:

If alkyl group is fixed then.

RF < RCI < RBr < RI

→ If there is possibility of formation of more than one alkene in elmination reaction, then that alkene will be preferred which contains largest number of alkyl groups. This rule is called Saytzeff’s rule. The order of reactivity of alkyl groups towards elimination is :

3° > 2° > 1°.

→ In case, an alkyl halide can form two aikenes, more substituted alkene (also known as Saytzeff product) is the major product and less substituted alkene falso known as Hofmann product) is the minor product.

→ However, remember that elimination reactions compete with substitution reactions. Elimination reactions dominate over substitution when strong Bronsted base (e.g., NH^–₂ Me₃CO^–, C₂H₅O^– etc.) is used and alkyl halide is 3° or 2°.

Summary of Substitution vs Elimination :

→ When the substrate is 1° halide. S_N2 reaction predominates except when a hindered strong base like Me3CO is used.

→ When the substrate in reaction 2° halide, S_N2 reaction occurs with weak bases like I^–, CN^–, RCOO^– etc., and E₂ resction occurs with strong bases like RO^–.

→ When the substrate is a 3° halide, S_N1 reaction is favoured at low temperature and in solvolysis (when no strong base/nucleophile is present and the solvent acts as a nucleophile/base); when a strong base (e.g., RO^–) is used E₂ reaction predominates.

Reaction with Metals :

→ Wurtz Reaction : When two molecules of alkyl halides react with metallic sodium in the presence of dry ether then alkane is formed containing double the number of carbon atoms present in the alkyl halides.

If two different alkyl halides are used then mixture of 3 possible alkanes are obtained.

→ Reaction with Lithium : Alkyl halides react with lithium in the presence of dry ether to form alkyl lithium.

→ Reaction with Zinc : Alkvl halides react with zinc metal to form dialkyl zinc (Franklend reagent)

→ ReactIon with Magnesium : When alkyl halide reacts with Mg metal in the presence of dry ether, then Grignard reagent is obtaind.

Alkyl magnesium halides are commonly called Grignard reagents.

Among alkyl halides, the order of reactivity is :

→ I > Br > Cl > F; since bromides are readily available and quite reactive, these are most oftenly used.

→ The reactivity of a metal with an alkyl halide depends on its reduction potential; the more easily a metal is reduced, the more reactive it is e.g., Mg > Zn.

→ The solvent used must be anhydrous because even a trace of water or alcohols react with metals to form insoluble hydroxide or alkoxide salts that coat the surface of the metal and thus prevent it from reacting with the alkyl halide. Moreover, organo metallic compounds are strong bases and react rapidly with ever weak proton sources to form hydrocarbons.

→ Reaction with Sodium Lead Alloy : When ethyl bromide reacts with sodium lead alloy. tetraethyl lead (TEL) is formed.

→ ReactIon of Alkyl Halides with Aromatic Hydro carbons (Elctrophilic Substitution ¡n Benzene): Benzene and other aromatic hydrocarbons react with alkyl halides, alkylene halides and other halogen derivatives in presence of anhydrous AlCl₃ to form higher aromatic hydrocarbons.

→ Formation of cumene is due to more stability of the 2° carbocation as compared to 1°.

→ Above reaction between alkyl halide and arenes can also be extended between alkyl halide and alkenes. e.g,

→ Reduction : Most alkyl halides may be reduced either via the Grignard reagent or directly with metal (usually zinc) and acid or with LiAlH₄ to produce an alkane.

→ In these reactions, zinc atoms transfer electrons to the carbon atom of the alkyl halide. Zinc is a good reducing agent because it has two electrons in an orbital far from the nucleus, which are readily donated to an electron acceptor.

Chemistry Notes