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Why are tertiary alcohol more reactive to haloacids than secondary and primary

Why are tertiary alcohol more reactive to haloacids than secondary and primary

Grade:12

12 Answers

Aarti Gupta
askIITians Faculty 300 Points
9 years ago
Alcohols are the class of organic compounds which have -OH as the functional group.
Reaction of alcohols with haloacids like HCl,HBr and HI involves the cleavage of C--OH bond of alcohols.For ex-
CH3CH2--OH + HBr ------> CH3CH2Br + H2O
However alcohols differ in the reactivity with haloacids, and the order is as follows---
Tertiary > secondary > primary
This is due to the electron releasing inductive effect of alkyl groups.The alkyl group release electrons and thus the e- density towards oxygen gets increased.As a result the polarity of C--O bond increases and this makes the cleavage of the bond between the C and O easier.Therefore,greater the number of alkyl groups on carbon,more easy would be the cleavage of C--O bond and thus greater would be the reactivity of alcohol.Since tertiary alocohol maximum no(which is 3) of alkyl groups it would be most reactive,than will be the secondary having 2 alkyl gropus and the last would be primary with only one alkyl gorup.
Pk
11 Points
9 years ago
but As polarity between c and o increases then the bond between c and o becomes more ionic and so its cleavage becomes difficult I think your general idea is correct but due to the inductive effect the polarity between carbon and oxygen decreases (not increases) and so it is easier to cleave the bond 
 
Pk
11 Points
9 years ago
Ma'am Pls tell me if I am correct
Aarti Gupta
askIITians Faculty 300 Points
9 years ago
Dear student inductive effect is of two types-
1) --I effect which is also known as e- withdrawing effect and
2).+I effct which is also known as e- releasing effect.
Alkyl groups show +I effect.So they releases e-s as a result of which the e- density towards C to which the alkyl group is attatched.But between C and O,due to electronegativity difference, e- density towards O will gets increased,which means that e-s are more towards O atom than the C.Threby Increased in polarity makes the cleavage of an bond more easier and not difficult.So cleavage will be more easier in tertiary alcohols because of most number of alkyl groups than the secondary which in turn more than primary,
Pk
11 Points
9 years ago
Can more stable tertiary carbocation be another reason 
Aarti Gupta
askIITians Faculty 300 Points
9 years ago
Dear student the order of stability of carbocation is –
tertiary > secondary > primary
This is due to the fact that greater the number of alkyl gorups on the carbon carrying +ive charge greater would be the dispersal of charge and hence more would be the stability of carbocation.So tertiary carbocation will be most stable and then secondary and at last primary.
But in case of reaction with haloacids reactivity order follow the above trend which depends on cleavage of carbon--oxygen bond.This in turn depends on the polarity order which is already explained.If stabilty of carbocation is taken as another reason than reactivity order will follow the opposite trend.As order of reactivity of carbocation is--
primary >secondary > tertiary
However the trend is in reverse order.Because ease of reaction of different types of alcohols with haloacids depends on the fact that how easy would be the breaking of carbon---oxygen bond,which is most fast with tertiary and the secondary and then primary as per the increase in polarity in the same order.
Anshu
13 Points
7 years ago
If the electron density on the oxygen atom of the OH bond increases,then why is the polarity of OH bond decreases and makes the cleavage of H+ion more difficult...instead if electron density increases,polarity should increases as now O-atom has greater electronegativity....plz make the necessary correction in my view
Pratik Agarwala
13 Points
6 years ago
As you see the mechanism of this reaction follows SN1 reaction so from we can conclude that rate of reaction will be 1degre
Aasoo
11 Points
6 years ago
respected maam,
i think ter>sec>pri is only true if reaction follows sn1 mechanism in whibh c-o bond is broken.as inductive effect is concern ter carbon connected to OH will bear less postive character because positive character produced on that carbon by electronegative oxygen is somewhat compensated by three electron donating methyl group.
in case where O-H bond is broken the order will be pri>sec>ter because in pri O-H bond is more polar and it is easy to break.
kitty
11 Points
6 years ago
In this reaction involving cleavage of -OH bond with the oxygen atom is to take up electron pair from the bond.The presence of alkyl group with +I effect,tends to increase electron density around oxygen atom.Hence,bond cleavage of -OH becomes easier and reactivity increases in the order 3°
james cook
15 Points
5 years ago
as the alkyl chain attached to OH group increases , electron releasing effect of C also increases . one C releases its electrons to second carbon ,then it releases them to next C and so on . the C attached to the OH releases its electrons to O atom thereby increasing electron density on O atom . As electrons were released by C to O ,there exists strong polarity between C and O , ultimately leading to decreased polarity between O-H. Due to the electron releasing effect, carbon develops high partial positive charge and so , even a weak nucleophile can easily attack on carbon ,forming it a carbocation and removing O-H group.HOPE THIS HELPS!!!
ankit singh
askIITians Faculty 614 Points
3 years ago
Tertiary alcohols are more reactive because the increased number of alkyl groups increases +I effect. So, the charge density on carbon atom increases and hence around oxygen atom. ... Hence, the cleavage of C−O bond becomes easier. So, a tertiary carbocation is formed which is more stable than secondary and primary

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