what is the condition for E1-CB reaction and E1 reaction ,are all E1 reaction E1-CB reaction

All E1 rxns are not E1cb. all E1cb are E1.

The conditions for an E1 reaction are a polar solvent, good leavin grp i.e weak nucleophile(e.g F^-, Cl^-) .

It usually takes place when a stable carbocation is formed ( benzyl, 3 degree etc.)

E1cb has all these conditions and also apart from these it needs to have a very strong withdrawing group (e.g F^-) .

All E1 rxns are not E1cb. all E1cb are E1.

The conditions for an E1 reaction are a polar solvent, good leavin grp i.e weak nucleophile(e.g F^-, Cl^-) .

It usually takes place when a stable carbocation is formed ( benzyl, 3 degree etc.)

E1cb has all these conditions and also apart from these it needs to have a very strong withdrawing group (e.g F^-) .

It depends upon the C-X/C-H bond strength.

1-If C-X > C-H ( in case of C-F), the predominant product will be E1cb.

2-If C-X < C-H (in case of C-I), the predominant product will be E1.

3-If C-X ~ C-H (in case of C-Cl, C-Br), then E1 or E2 wud take place. Now to find the major product between E1 and E2, we see the hydrocarbon used-

Hope it helps you.

 (i)- If hydrocarbon is tertiary- E1

(ii)-If hydrocarbon is primary- E2

(iii)-If hydrocarbon is secondary- Both E1 and E2

tarin how can C-H bond be smaller than C-F bond is it your typing error

cs9192 F^- is a weak nucleophile how can it be a strong withdrawing group

E1 cb reaction is not very usual, but it takes place when a weak leaving group like fluoride ion is present. also E1 suffers from isotopic effect whereas E1 cb suffers from isotopic exchange(H and D)