Properties of Haloalkanes – Physical and Chemical

Some of the important physical and chemical properties of haloalkanes are as follows:

Physical properties of haloalkanes

Physical state of Haloalkanes

  • Chloride, Bromide, Floride of methane and Chloride of ethane are Gases at room temperature.
  • Rest alkyl halide up to $C_{18}$ are colorless liquid.
  • Alkyl halides beyond $C_{18}$ are colorless solid.

Odour of Haloalkanes

Many volatile halogen compounds have a sweet smell.

Solubility of Haloalkanes

Alkyl halides are polar in nature but not soluble in polar solvents like water. As they can’t form hydrogen bonds.

Haloalkanes are soluble in organic solvents like benzene, ether etc.

Density of Haloalkanes

The density of alkyl halide decreases, as the size of alkyl group increases.

Density of Haloalkanes for the same alkyl group,

\[RF < RCl < RBr < RI\]

Melting and Boiling Points of Haloalkanes

The boiling point of chlorides, bromides, and iodides are considerably higher than the hydrocarbons of comparable molecular mass.

For the same alkyl group,

The boiling point increase as the atomic mass of the halogen increases.

\[RI > RBr > RCl > RF\]

For the same halogen atom,

The boiling point increases with increase in the size of the alkyl groups.

\[CH_{}X < CH_{3}CH_{2}X < CH_{3}CH_{2}CH_{2}X < CH_{3}CH_{2}CH_{2}CH_{2}X\]

For isomeric alkyl halides,

The boiling point decreases with branching due to due to the decrease in surface area of alkyl halide.

\[CH_{2}CH_{2}CH_{2}CH_{2}Cl > CH_{3}CH_{2}CHClCH_{3} > (CH_{3})_{2}CHCH_{2}Cl > (CH_{3})_{3}CCl\]

Boiling point increases with the decrease in the number of halogen atom.

\[CH_{3}Cl < CH_{2}Cl_{2} < CHCl_{3} < CCl_{4}\]

Dipole moment of Haloalkanes

The carbon-halogen bond length increase as the halogen atoms become bigger.

\[C-F < C-Cl < C-Br < C-I\]

The larger halogens have longer bonds but weaker electronegativities in order.

We know,

\[ \mu_{\text{ Dipole moment}} = e \times d\]

e = charge, d = bond length.

The bond dipole moment increases in the order:

\[C-I < C-Br < C-F < C-Cl \]

\[CH_{3}-I < CH_{3}-Br < CH_{3}-F < CH_{3}-Cl \]

Chemical Properties of Haloalkanes

Important chemical reaction types of haloalkanes are:

  1. Nucleophilic substitution reaction.
  2. Elimination reaction.
  3. Reaction with metals.
  4. Reduction of haloalkanes.

Nucleophilic substitution reactions of Haloalkanes or Alkyl halides (R-X)

In this reaction, a nucleophile attack to the alkyl halide and react with the electrophilic carbon atom and substitution occurs. The halogen atom which departs with its bonding pair of electrons is called leaving the group.

Generalized Nucleophilic substitution reaction of alkyl halides,

\[\underbrace{R-X}_{\text {X = Leaving group}} + Nu^{-} \xrightarrow {}R-Nu + X^{-}\]

The leaving group ability of halide ions follows the order:

\[I^{-} > Br^{-} > Cl^{-} > F^{-}\]

There occurs two type of nucleophilic substitution reaction:

  1. $S_{N}1$ type nucleophilic substitution reaction.
  2. $S_{N}2$ type nucleophilic substitution reaction.

$S_{N}1$ type (Unimolecular nucleophilic substitution reaction)

In this the rate of nucleophilic substitution reaction depends upon the concentration of the substrate not on the nucleophile. It is of one step reaction and follows the first order kinetics.

Rate equation,

\[ Rate = K [R-X] \]

It gives 50:50 mixture of two enantiomers, i.e. a racemic mixture.

The carbocations are the intermediates in $S_{N}1$ reaction. Thus, more the stability of carbocation, more easily it is formed and hence, faster will be the rate of reaction.

It is favored by weaker nucleophiles and polar protic solvents. Better the leaving group faster be the reaction.

Decreasing order of stability of carbocation,

\[\text{Allyl} \approx \text{Benzyl carbocation} > 3^{\circ} \text{carbocation} > 2^{\circ} \text{carbocation}> 1^{\circ} \text{carbocation}\]

Allyl and benzyl carbocations are more stable due to resonance.

Reactivity of haloalkanes toward $S_{N}1$ decreases in the same order:

\[\text{Allylic halides} \approx \text{Benzylic halides} > 3^{\circ} \text{alkyl halide} > 2^{\circ} \text{alkyl halide}> 1^{\circ} \text{alkyl halide}\]

Factors affecting $S_{N}1$ reactions,

  1. It does not depends upon the concentration of nucleophile. Thus, reaction not affected by nucleophilicity.
  2. Good leaving groups like Cl, Br, I proceed reaction faster.
  3. The reaction is favored by solvents of high ionizing power(polar protic solvents). It helps to pull the leaving group (X) out of the molecule (R-X)

$S_{N}2$ type (Bimolecular nucleophilic substitution reaction)

The depends upon the concentration of the two molecules (i.e R-X and Nucleophile).It follows second order kinetics and commonly seen in primary alkyl halides and alcohols.

Relative reactivity of alkyl halides toward $S_{N}2$  reaction,

Methyl halides > Primary halides > Secondary halides > tertiary halides

Factors affecting $S_{N}2$  reactions,

  1. Steric effects – When bulky substituents on or near the C-atom bonded to halogen tends to hinder the approach of the nucleophile to that C-atom and makes the reaction difficult.
  2. Nucleophilicity – A stronger nucleophile(like $R_{3}P, I^{-}, HS^{-}, RS^{-}$) favor this reaction while weak nucleophile (like RCOOH) fails to promote the $S_{N}2$  reaction.
  3. Leaving group ability – Better the leaving group, faster is the reaction.Halide reactivity follows this sequence $F^{-} > Cl^{-} > Br^{-} > I^{-}$.
  4. Effect of solvent – It is favored by aprotic solvents such as DMSO (dimethylsulphoxide), DMF (dimethylformamide), or HMPT (hexamethylphosphorotriamide).

Sodium alkoxide ($R{}’O^{-}Na^{+}$) reaction with alkyl halides

\[CH_{3}-I + \underbrace{Na^{+}O^{-}CH_{3}}_{\text{Sodium alkoxide}} \xrightarrow{\Delta} \underbrace{CH_{3}-OCH{3}}_{\text{Ether}}+NaI\]

 $NH_{3}$ reaction with alkyl halides

\[CH_{3}CH_{2}Br + H-NH_{2} \xrightarrow[C_{2}H_{5}OH]{\Delta} \underbrace{CH{3}CH{2}NH{2} }_{\text{primary amine}}+ HBr\]

$AgCN$ reaction with alkyl halide

\[CH_{3}-I + AgCN \xrightarrow {C_{2}H_{5}OH/H_{2}O} \underbrace{R-NC}_{\text{Isonitrile}} + AgX\]

$KNO_{2}$ reaction with alkyl halides

\[CH_{3}CH_{2}Br + K^{+}-O-N=O \xrightarrow {\Delta}CH_{3} \underbrace{CH_{2}-O-N=O}_{\text{Alkyl nitrite}} + KBr\]

$AgNO_{2}$ reaction with alkyl halides

\[CH_{3}CH_{2}Br + AgO-\ddot{N}=O \xrightarrow[C_{2}H_{5}OH/H_{2}O]{\Delta} \underbrace{CH_{3}CH_{2}-ON\rightarrow{}O}_{\text{Nitroalkane}} + AgBr\]

Sodium alkynide ($Na^{+}C \equiv R{}’$) reaction with alkyl halides

\[CH_{3}CH_{2}Br + Na^{+}-C\equiv CR \rightarrow \underbrace{CH_{3}CH_{2}-C \equiv C-R}_{Alkyne} + NaBr\]

Elimination reaction of Haloalkanes or Alkyl halides (R-X)

In elimination reaction, two froups on adjacent atoms or on same atom are lost and a double bond is formed.

The product is only an alkene or alkyne. These are given by the compound those have nucleophile as a leaving group ($X^{-},\bar{O}H, C_{2}H_{5}O^{-}, N_{2}^{\oplus}$).


\[ C_{2}H_{5}O^{-} + CH_{3}C(CH_{3})BrCH_{3} \xrightarrow {}C_{2}H_{5}OH + H_{2}C=C(CH)_{3}+Br^{-}\]

Reaction of haloalkanes or Alkyl halides (R-X) with metals

Reaction of alkyl halides with magnesium

Alky halide reacts with Mg in the presence of dry diethyl ether ($C_{2}H_{5}-O-C_{2}H_{5}$) and forms Grignard reagent ($RMgX$, R can be aryl or alkyl).

\[CH-{3}X + Mg \xrightarrow{Ether} CH_{3}MgX\]

X=Cl, Br, I

Reactivity with Mg to form Grignard reagent in order $RI > RBr > RCl$

For a given halide the reactivity order of ‘R’ is

\[3^{\circ} > 2^{\circ} > 1^{\circ}\]

Wurtz reaction of alkyl halides

The ethereal solution of an alkyl halide is treated with sodium to produce higher alkanes (even number of carbon atom).

Methane can’t be prepared with this method.

A single product is obtained when symmetrical alkyl halides are used.

\[2CH_{3}-Br + 2Na \xrightarrow{Ether}CH_{3}CH_{3} + 2NaBr\]

If different alkyl halides are used the product is the non-separable mixture of alkanes.

\[CH_{3}CH_{2}Br + 2Na \xrightarrow{Ether} CH_{3}CH_{2}CH_{3} + CH_{3}CH_{3} + C_{2}H_{5}-C_{2}H_{2} + 2NaBr\]

Corey-House synthesis of alkyl halides

Alkyl halide react with lithium dialkyl cuprate ($R_{2}CuLi$) to form a new alkane.

It is a good method to for the odd number of alkane and methane can’t be formed by this method.

\[\underbrace{R-X}_{\text{Alkyl halide}} + \xrightarrow [\text{Diethyl ether}]{\text{Li}} RLi \xrightarrow{\text{CuI}} \underbrace{R_{2}CuLi}_{\text{Gilman reagent}} \xrightarrow[R{}’X]{} R-R{}’ + RCu + LiX\]

$R{}’$ should be methy, $1^{\circ}$alkyl or $2^{\circ}$ cycloalkyl.

Reduction of haloalkanes

Haloalkanes reduced to alkanes.

Reducing can be used,

$Zn/CH_{3}COOH$, Zn/HCl, Zn/NaOH, {Zn-Cu couple}, $LiAlH_{4}, NaBH_{4}$ etc.

\[R-X \xrightarrow{\text{Reduction}}R-H\]