Haloalkanes

Haloalkanes:-

Alkyl halides are the compounds in which a halogen atom is attached to carbon. For example, methyl chloride (CH₃Cl), ethyl bromide (C₂H₅Br), etc.

They have the general formula:

R—X,

Where, R= alkyl group, X = F, Cl, Br, or I (functional group).The halogen atom bonded to carbon is the functional group of alkyl halides. Alkyl halides are classified as Primary (1^◦), Secondary (2^•), or Tertiary (3^•), depending upon whether the X atom is attached to primary, secondary, or a tertiary carbon.

Nomenclature of haloalkanes: – Alkyl halides are named in two ways:

1) Common System: –

In this system the alkyl group attached to the halogen atom is named first. This is then followed by an appropriate word chloride, bromide, or fluoride. Notice that the common names of alkyl halides are two-word names.

CH₃—Br                     CH₃CH₂CI                              CH₃—CH (Br)—CH₃
Methyl bromide          Ethyl chloride                            Isopropyl bromide

2) IUPAC System: –

The IUPAC names of alkyl halides are obtained by using the following rules:

a) Select the longest carbon chain containing the halogen atom and name the alkyl halide as a derivative of the corresponding hydrocarbon.

b) Number the chain so as to give the carbon carrying the halogen atom the lowest possible number.

c) Indicate the position of the halogen atom by a number and by the fluoro-, chloro-, bromo-, or iodo.

d) Name other substituents and indicate their positions by numbers.

The examples given below show how these rules are applied. Notice that the IUPAC names of alkyl halides are one-word names.

CH₃CH₂Br       CH₃CH₂CH₂—I        CH₃—CH (CI) —CH₃      CH₃—CH(CH₃)—CH₂—CH₂Br
Bromoethane     1 -Iodopropane    2-Chloropropane              1-Bromo-3-methylbutane

Nature of C—X bond:-

Let us consider methyl chloride (CH₃-Cl) for explaining the orbital make up of alkyl halides. In methyl chloride, the carbon atom is sp³ hybridised. The chlorine atom has a half-filled p orbital in its valence shell. The C—Cl bond is formed by the overlap of an sp³ orbital of carbon and the half-filled p orbital of chlorine atom. Each C—H bond is formed by the overlap of an sp³ orbital of carbon and s orbital of hydrogen. All bond are σ-bonds. The bond angle is approximately tetrahedral.

Physical Properties: –

• CH₃C1, CH₃Br, CH₃F, and CH₃CH₂C1 are gases at room temperature. Other alkyl halides up to C₁₈ are colourless liquids. Those beyond C₁₈ are colourless solids.
• Alkyl halides are insoluble in water but soluble in organic solvents. The insolubility in water is due to their inability to form hydrogen bonds with water.
• Alkyl bromides and iodides are denser than water. Alkyl chlorides and fluorides are lighter than water.
• Alkyl halides have higher boiling points than alkanes of comparable molecular weight. For a given halogen atom the boiling points of alkyl halides increase with the increase in the size of the alkyl group. For a given alkyl group, the boiling points of alkyl halides follow the order RI> RBr> RC1 > RF.

Chemical Properties: –

Alkyl halides are very reactive compounds. They undergo substitution, elimination, and reduction reactions. Alkyl halides also react with metals to form organometallic compounds.

Substitution Reactions: –

The carbon-halogen bond in alkyl halides is polar because of the high electronegativity of the halogen atom relative to carbon. The carbon atom is therefore a good target for attack by nucleophiles (electron rich species).In fact, the nucleophilic substitution reactions are the most common reactions of alkyl halides. They may be represented as:

Alkyl halides can undergo nucleophilic substitution by two mechanisms:

S_N² Mechanism: – S_N² stands for bimolecular nucleophilic substitution. In this mechanism, the attack of the nucleophile and the ejection of the halide ion takes place simultaneously. The reactants on their way to products pass through a transition state where the C—OH bond is half-formed and the C—X bond is half-broken. For example, hydrolysis of methyl bromide by aqueous NaOH. The reaction and the transition state are shown below.

S_N¹ Mechanism: – S_N¹ stands for unimolecular nucleophilic substitution. This mechanism involves two steps. e.g. hydrolysis of tert-butyl bromide with aqueous NaOH.

Step-1) The alkyl halide ionises to give a planar carbonium ion. The carbonium ion is planar because the central positively charged carbon atom is sp² hybridized.

tert-Butyl bromide            Carbonium ion

Step-2) The nucleophile can attack the planar carbonium ion from either side to give the product.

Note:

• Primary alkyl halides undergo substitution by S_N² mechanism.
• The tertiary alkyl halides undergo substitution by S_N¹ mechanism. This is because the attack of the nucleophile on the crowded tertiary alkyl halide is quite difficult.

                                                  Easy attack                             Difficult attack

(Primary alkyl halide)              (Tertiary alkyl halide)

• The secondary alkyl halides may undergo nucleophilic substitution by either S_N¹or S_N² mechanism depending on the solvent. S_N¹ mechanism will predominate if an ionising solvent is present. S_N² mechanism will predominate if ionising solvent is absent.

Nucleophilic Substitution Reactions:-

Some of the important nucleophilic substitution reactions of alkyl halides-

1) Reaction with aq. KOH: – Alkyl halides react with aqueous solution of KOH to form alcohols. The halogen atom is substituted by- OH group.

CH₃I  +  aq.KOH ——> CH₃-OH   + KI

Methyl iodide             Methyl alcohol

CH₃CH₂Br  +  aq.KOH ———–> CH₃CH₂OH   + KBr

Ethyl bromide                                Ethyl alcohol

2) Reaction with Moist Silver Oxide: – Alkyl halides on treatment with a suspension of silver oxide in moist ether produce alcohols.

Ag₂O  + H₂O ——–> AgOH

CH₃CH₂Br  +  AgOH ——-> CH₃CH₂OH   + AgBr

Ethyl bromide                         Ethyl alcohol

3) Reaction with sodium alkoxides: – Alkyl halides react with sodium alkoxide (RONa) to form ether. Sodium alkoxides are prepared by dissolving metallic sodium in excess of the appropriate alcohol. e.g.

CH₃CH₂OH +  Na ——-> CH₃CH₂ONa  + H₂

Sodium ethoxide

CH₃CH₂Br  +  CH₃CH₂ONa ——-> CH₃CH₂OCH₂CH₃   + NaBr

Ethyl bromide                                     Diethyl ether

4) Reaction with Ammonia: – When an alkyl halide is heated with an alcoholic solution of ammonia in a sealed tube, alkylation of ammonia takes place. A mixture of different classes of amines results.

CH₃CH₂Br + alco.NH₃ ——–Δ——> CH₃CH₂NH₂ + HBr

Ethyl bromide                                         Ethylamine (1°)

CH₃CH₂NH₂ + CH₃CH₂Br ————> (CH₃CH₂)₂NH + HBr

Diethylamine (2°)

(CH₃CH₂)₂NH + CH₃CH₂Br ———–> (CH₃CH₂)₃N + HBr

Triethylamine (3°)

(CH₃CH₂)₃N + CH₃CH₂Br ————> (CH₃CH₂)₄⁺NH₃^–Br

Tetraethylammonium bromide (4°)

5) Reaction with Sodium Cyanide: – Alkyl halides react with sodium cyanide in a suitable solvent (generally aqueous ethanol) to form alkyl cyanides or nitriles. Halogen atom is replaced by —CN group.

CH₃CH₂—Br + aq. NaCN (ethanol) ——> CH₃CH₂—CN + NaBr

Ethyl bromide                                               Ethyl cyanide

Alkyl halides react with silver cyanide to form isocyanides.

CH₃CH₂—Br + AgCN ———–> CH₃CH₂—NC + AgBr

Ethyl bromide                                  Ethyl Isocyanide

6) Reaction with RCOOAg: – When an alkyl halide is treated with an alcoholic solution of the silver salt of a carboxylic acid, an ester is formed.

CH₃COOAg  + Br —CH₂CH₃ (Alco) ——–> CH₃COOCH₂CH₃ + AgBr

Silver acetate    Ethyl bromide                                   Ethyl acetate

7) Reaction with AgNO₃:- Alkyl iodide reacts with silver nitrate to form nitroalkanes.

CH₃CH₂—I + AgNO₃ ———->  CH₃CH₂—NO₂ + AgI

Ethyl Iodide                                      Nitroethane

8) Reaction with Acetylides: – Alkyl halides react with sodium acetylides to form higher alkynes.

CH₃—Br + Na—C≡C—H ———–> CH₃—C≡C—H + NaBr

Sodium acetylides                 Propyne

9) Reaction with K₂S: – Alkyl halides react with potassium sulphide to form dialkyl sulphide.

2CH₃CH₂—I + K₂S ————>  CH₃CH₂—S—CH₂CH₃ + KI

Ethyl Iodide                                    Diethyl sulphide

10) Reaction with KSH: – Alkyl halides react with alcoholic potassium hydrosulphide to form thiols. The halogen atom is substitute by –SH group.

CH₃CH₂—I + KSH (Alco) ————> CH₃CH₂—SH + KI

Ethyl Iodide                                                Ethanethiol

Note:

1) Reaction with alco. KOH: – Alkyl halides react with alcoholic potassium hydroxide to form alkene. The reaction involves the elimination of HX from the alkyl halides and is called dehydrohalogenation reaction.

CH₃CH₂—Br + KOH (alco) ——-Δ—–> CH₂=CH₂ + KBr + H₂O

Ethyl bromide                                              Ethylene

2) Reaction with Mg: – Alkyl halides react with magnesium metal in dry ether to form Grignard reagent (alkyl magnesium halide).

CH₃—I        +      Mg  —-Dry ether—->CH₃MgI

Methyl iodide                                    Methyl magnesium iodide

3) Reaction with Li: – Alkyl halides react with lithium in dry ether to form alkyl lithium, which behave in the same way as Grignard reagent, but with increasing reactivity.

CH₃CH₂—Br +  Li —-Dry ether—-> CH₃CH₂Li       + LiBr

Methyl iodide                                      Ethyl lithium

4) Wurtz Reaction: – Alkyl halides react with metallic sodium in dry ether to form alkanes with double the number of carbon atoms.

CH₃CH₂—Br +    Na +    Br—CH₂CH₃ —–Ether—->CH₃CH₂—CH₂CH₃ + NaBr

Ethyl Bromide                Ethyl Bromide                                 Butane