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Bond strenghts

I am just wonderign if this is the right answer going from the strongest bond to weakest...

[ C-Cl ]> [ C-Br] > [ C-C ]

thanks for your help. :yes:

Reply 1

shengoc

12

Bond strength in main group is directly proportional to bond length.

C-Br has longest bond as it is 2p-4p bond

C-Cl is 2p-3p bond

C-C is 2p-2p bond

Well, essentially when bonding, these will be hybrids, but according to these, strongest bond would be C-C, then C-Cl and then C-Br. That is why forming C-C usually drive reaction forward; also, think why Br is good leaving group in halocompounds such that sn1 or sn2 can occur easily - weak bonds.

Reply 2

LadySmythe

19

For A level I think its for atom sizes as well as usual bond lengths

For instance Cl is smaller than Br, so there will be more attraction to the nucleus and less electron shielding, so the bond length will be less and thus be a stronger bond. I think the bond length increases down the halogens or something like that.

I think thats right anyway.

Reply 3

shengoc

12

Well yeah, A-Level tends to be over-simplified sometimes.

Reply 4

LadySmythe

19

Yea, I've noticed a lot of undergrads and grads saying that.

Reply 5

Janka3112

OP

2

shengoc

Bond strength in main group is directly proportional to bond length.

C-Br has longest bond as it is 2p-4p bond

C-Cl is 2p-3p bond

C-C is 2p-2p bond

Well, essentially when bonding, these will be hybrids, but according to these, strongest bond would be C-C, then C-Cl and then C-Br. That is why forming C-C usually drive reaction forward; also, think why Br is good leaving group in halocompounds such that sn1 or sn2 can occur easily - weak bonds.

Thanks a lot. :woo:

Reply 6

Orihime

2

shengoc

Bond strength in main group is directly proportional to bond length.

C-Br has longest bond as it is 2p-4p bond

C-Cl is 2p-3p bond

C-C is 2p-2p bond

Well, essentially when bonding, these will be hybrids, but according to these, strongest bond would be C-C, then C-Cl and then C-Br. That is why forming C-C usually drive reaction forward; also, think why Br is good leaving group in halocompounds such that sn1 or sn2 can occur easily - weak bonds.

Shouldn't the C-Br bond be stronger than c-cl as, the cl is more electronegative so the carbon will be more positive and thus more susceptible to undergo nucleophilic substitution?

Reply 7

shengoc

12

Orihime

Shouldn't the C-Br bond be stronger than c-cl as, the cl is more electronegative so the carbon will be more positive and thus more susceptible to undergo nucleophilic substitution?

That is a catch really, you'd think electronegativity is the dominant factor but here are the real book values,
MeF 109 (452)
MeCl 84 (352)
MeBr 70 (293)

Similarly, HBr is a stronger acid than HCl for the same reason despite electronegativity reasoning.

HF 135 (570)
HCl 103 (431)
HBr 87 (366)

You can check out these values on the wiki page; http://en.wikipedia.org/wiki/Bond_energy

I tend to think that electronegativity is a subtle effect, whereas the overlap is more physical(to me) hence more dominant factor in determining the bond strength.

Reply 8

jm01980

3

Electronegativity does not determine bond strength. Only the force of attraction can determine that. Hence cl is smaller so bigger force of attraction = stronger bond. What electronegative does affect is how reactive a molecule is, (bond strength also affects how reactive a molecule is)

Original post by jm01980

Electronegativity does not determine bond strength. Only the force of attraction can determine that. Hence cl is smaller so bigger force of attraction = stronger bond. What electronegative does affect is how reactive a molecule is, (bond strength also affects how reactive a molecule is)

OP posted this on 9th December 2009.

I imagine that he/she's chuffed to rocks that he/she's finally got a useful answer, well done.

Bond strenghts

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