Question

4. What is the OBSERVEDD change in the rate of reaction with AgNO3 in alcohol caused by each of the following? (Cite experimental evidence.) (A) Nature of halogen (Cl, Br) (B) Extent of branching of RX (C) Steric hindrance

Answer 1

Silver nitrate solution is used to find out which halogen is present in a particular compound. The observations for this test depends on several factors. Here are the 3 listed factor that has a direct influence on the observations of this test.

a) Nature of Halogens- Different type of halogens present in the compound gives rise to different observations. Observations that are made in the presence of different halogens are summarized in a tabular form below-

Halide Type	Observation
Cl-	white precipitate
Br-	a very pale cream precipitate
I-	a very pale yellow precipitate

Not only the color changes in the precipitate formed are observed but also it affects the reaction rates if the alkyl part of the RX is kept constant and halides are changed.

For example:

1. A primary iodo compound produces a precipitate quite quickly.
2. A primary bromo compound takes longer to give a precipitate.
3. A primary chloro compound will take a much longer time to give any precipitate.

The order of reactivity depends upon the strengths of the carbon-halogen bonds. The carbon-iodine bond is the weakest and the carbon-chlorine the strongest of the three bonds. In order for a halide ion to be produced, the carbon-halogen bond must be broken. The weaker the bond, the easier it is to cleave the carbon-halogen bond.

b) Extent of branching in RX - If the halogen atom is kept constant, then the observations change considerably with the change in R (alkyl) part of the RX. The extent of branching in the alkyl part directly affects the reaction rate.

1. The tertiary halide produces a precipitate almost instantly.
2. The secondary halide gives a slight precipitate after a few seconds. The precipitate thickens up with time.
3. The primary halide may take considerably longer to produce a precipitate.

c) Steric Hindrance - Steric hindrance also play a major role in the reaction. In the mechanisms for these reactions, a lone pair on the alcohol molecule attacks the slightly positive carbon atom attached to the halogen and cleave the C-X bond. So the presence of a more bulky group in the alkyl part will hinder the approach of alcohol molecule to the slightly positive carbon atom attached to the halogen. Thus, in turn, will decrease the reaction rate considerably. Hence lesser the steric hindrance in the alkyl part easier is the cleavage of C-X bond.