The alkyl halides are highly reactive, the order of reactivity is,
Iodide > Bromide > Chloride (Nature of the halogen atom)
Tertiary > Secondary > Primary (Type of the halogen atom)
Amongst the primary alkyl halide, the order of reactivity is : CH3X > C2H5X > C3H7X, etc.
The high reactivity of alkyl halides can be explained in terms of the nature of C — X bond which is highly polarised covalent bond due to large difference in the electronegativities of carbon and halogen atoms. The halogen is far more electronegative than carbon and tends to pull the electrons away from carbon, i.e.,halogen acquires a small negative charge and carbon a small positive charge.
This polarity is responsible for reactions,
(i) Nucleophilic substitution reactions (ii) Elimination reactions
Example 1: Alkyl halides can be converted into Grignard reagents by
(a) Boiling them with Mg ribbon in alcoholic solution
(b) Warming them with magnesium powder in dry ether
(c) Refluxing them with MgCl2 solution
(d) Warming them with MgCl2
Example 2: Which is not present in Grignard reagent
(a) Methyl group (b) Magnesium
(c) Halogen (d) —COOH group
Example 3: The reactivity of ethyl chloride is
(a) More or less equal to that of benzyl chloride
(b) More than that of benzyl chloride
(c) More or less equal to that of chlorobenzene
(d) Less than that of chlorobenzene
Nucleophilic substitution (SN) reactions
The Cδ+ site is susceptible to attack by nucleophiles (An electron rich species).
Nu– + R – X ————> Nu – R + X–
Example 1: Aryl halide is less reactive than alkyl halide towards nucleophilic substitution because
(a) Less stable carbonium ion
(b) Due to large C — Cl bond energy
(c) Inductive effect
(d) Resonance stabilization and sp2 - hybridisation of C attached to halide
Example 2:R — X + NaOH ————> ROH + NaX
The above reaction is classified as
(a) Nucleophilic substitution
(b) Electrophilic substitution
Reactions of Alkyl Halide
(a) Reaction with alkoxides or dry silver oxide :
(b) Reaction with sodium or potassium hydrogen sulphide :
(c) Reaction with alcoholic potassium cyanide and silver cyanide :
(d) Reaction with potassium nitrite or silver nitrite :
(e) Reaction with ammonia :
Elimination reactions of Alkyl Halides
The positive charge on carbon is propagated to the neighbouring carbon atoms by inductive effect. When approached by a strongest base (B), it tends to lose a proton usually from the β-carbon atom. Such reactions are termed elimination reactions. They are also E1 and E2 reactions.
E1 reaction :
As the above reactions involve leaving of X-, the reactivity of alkyl halides (Same alkyl group, different halogens) should be limited with C - X bond strength.
Type of bond C - I C - Br C - CI
The breaking of the bond becomes more and more difficult and thus, the reactivity decrease.
The order of reactivity (Tertiary > Secondary > Primary) is due to +I effect of the alkyl groups which increases the polarity of C - X bond.
The primary alkyl halides undergo reactions either by SN2 or E2 mechanisms which involve the formation of transition state. The bulky groups cause steric hinderance in the formation of transition state. Therefore, higher homologues are less reactive than lower homologues. CH3 X > C2H5X > C3H7X , etc.
Examples of elimination reaction
(a) Dehydrohalogenation :
In this reactions, ether is a by-product as potassium ethoxide is always present in small quantity.
C2H5 Br + KOC2H5 ——→ C2H5 - O - C2H5 + KBr
(b) Action of heat :
The decomposition follows the following order,
Iodide > Bromide > Chloride (When same alkyl group is present) and
Tertiary > Secondary > Primary (When same halogen is present).
(a) Substitution (b) Elimination
(c) Addition (d) Rearrangement
An ether solution of an alkyl halide (Preferably bromide or iodide) gives an alkane when heated with metallic sodium.
2RX + 2Na ——→ R - R + 2NaX
Example 1: Reaction C2 H5I + C5H11I + 2Na → C2H5 - C5H11 + 2NaI is called
(a) Hoffmann's reaction
(b) Dow's reaction
(c) Wurtz's reaction
(d) Riemer-Tiemann's reaction
Example 2: In Wurtz reaction alkyl halide react with
Reactions Of Alkyl Halides
Alkyl halides form Grignard reagent when treated with dry magnesium powder in dry ether.
Grignard reagents are used for making a very large number of organic compounds.
Reaction with other metals : Organometallic compounds are formed.
- When heated with zinc powder in ether, alkyl halides form dialkyl zinc compounds. These are called Frankland reagents.
- When heated with lead-sodium alloy, ethyl bromide gives tetra ethyl lead which is used an antiknock compound in petrol.
4C2H5Br + 4 Pb ( Na) ———→ ( C2H5)4 Pb + 4 NaBr + 3Pb
- Reaction with lithium : Alkyl halides react with lithium in dry ether to form alkyl lithiums.
Alkyl lithiums are similar in properties with Grignard reagents. These are reactive reagents also.
Friedel-Craft's reaction :
Substitution (Halogenation) : Alkyl halides undergo further halogenation in presence of sunlight, heat energy or peroxide.
Example 1: A new carbon-carbon bond formation is possible in
(a) Cannizzaro reaction (b) Friedel-Craft's alkylation
(c) Clemmensen reduction (d) Reimer-Tiemann reaction