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organic chem a level

Which compound has the fastest rate of reaction with potassium cyanide to form
pentanenitrile?

A 1-bromobutane
B 1-chlorobutane
C 1-fluorobutane
D 1-iodobutane

I found out the answer is D but I originally put A (as a guess) can anyone tell me why?

Thanks :smile:
Reply 1
Avatar for zlan
zlan
11
I'd say something like the cl more electronegative than iodine therefore the C-Cl bond is stronger than C-I bond remember one that reacts quickest has weakest bonds
Hope this helps
Reply 2
Avatar for MoleChem
MoleChem
3
Rate of reaction of haloalkanes with nucleophiles depends on the C-X bond strength, where X is any group 7 element.

Going down group 7 the C-X bond gets weaker. During the mechanism the C-X bond has to break for CN nucleophile to bond to form C-CN.

Therefore 1-iodobutane, with a C-I bond, has the weakest bond, and therefore breaks more easily, and forms product faster (so has faster rate).
Original post by MoleChem
Rate of reaction of haloalkanes with nucleophiles depends on the C-X bond strength, where X is any group 7 element.

Going down group 7 the C-X bond gets weaker. During the mechanism the C-X bond has to break for CN nucleophile to bond to form C-CN.

Therefore 1-iodobutane, with a C-I bond, has the weakest bond, and therefore breaks more easily, and forms product faster (so has faster rate).

Do you know why the C-X bond where X is a halogen gets weaker going down the group?
Thanks.
Reply 4
Avatar for zlan
zlan
11
As you go down the group more shells/shielding the less able the nucleus is to attract pair of electrons in covalent bond also the less electronegative the weaker the polar C-X bond
Original post by zlan
As you go down the group more shells/shielding the less able the nucleus is to attract pair of electrons in covalent bond also the less electronegative the weaker the polar C-X bond

thank you!!!!

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