info@urfaculty.com

www.urfaculty.com

Govt. Job/ Sarkari Naukri, NEET, CAT, Bank Exam & Study materials

Friday, 24 February 2017

Alcohols, Phenols and Ethers - III


Chemical properties of phenols:

I.Reactions involving cleavage of O–H bond: Alcohols react as nucleophiles:

a) Reaction with metals
b) Esterification reaction
II. Other chemical reactions of phenols:
III. Acidic nature of phenol and alcohol:

a) Phenol > H2O > Primary alcohol > Secondary alcohol > Tertiary alcohol

The acidic character of alcohols is due to the polar nature of O–H bond. Alkyl group is an electron-releasing group (–CH3, –C2H5) or it has electron releasing inductive effect (+I effect). Due to +I effect of alkyl groups, the electron density on oxygen increases. This decreases the polarity of O-H bond. And hence the acid strength decreases.

b) Phenol is more acidic than alcohol:

In phenol, the hydroxyl group is directly attached to the sp2 hybridised carbon of benzene ring which acts as an electron withdrawing group whereas in alcohols, the hydroxyl group is attached to the alkyl group which have electron releasing inductive effect. In phenol, the hydroxyl group is directly attached to the sp2 hybridised carbon of benzene ring whereas in alcohols, the hydroxyl group is attached to the sp3 hybridised carbon of the alkyl group. The sp2 hybridised carbon has higher electronegativity than sp3 hybridised carbon. Thus, the polarity of O–H bond of
phenols is higher than those of alcohols. Hence, the ionisation of phenols is higher than that of alcohols.
The ionisation of an alcohol and a phenol takes place as follows:

In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised.

The delocalisation of negative charge makes phenoxide ion more stable and favours the ionisation of phenol. Although there is also charge delocalisation in phenol, its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion.

c) In substituted phenols, the presence of electron withdrawing groups such as nitro group enhances the acidic strength of phenol. On the other hand, electron releasing groups, such as alkyl groups, in general, decreases the acid strength. It is because electron withdrawing groups lead to effective delocalisation of negative charge in phenoxide ion.

Click here to view complete Chemistry Free Study Materials and Notes for NEET Preparation

Click here to view complete Physics Free Study Materials and Notes for NEET Preparation

Click here to view complete Biology Free Study Materials and Notes for NEET Preparation

Click here to take NEET online practice tests by UrFaculty.com

No comments:
Write comments

Hey, we've just updated our test package. Check'it out here - https://goo.gl/r7FuPT
Join Youth Apps