BS Section Test 3 Bond length

[SaintJude]

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Yes, I'm still reviewing the same section test. Kaplan's section test aren't easy!

Which of the following statements explains the single bond length in 1,3-butadiene?

Compound*****Bond length (Å)
CH3—CH3************1.54
CH2=CHCH3**********1.50
CH2=CH—CH=CH2**1.48

I. The pi bonding orbital is delocalized across the single bond.
II. The bond length decreases as the number of carbons in the chain increase.
III. There is more s character in the 1,3 butadiene single bond.

Answer is : I &III
Can someone explain?

Only one or more can be correct.

 

[Jarudy]

There is overlap between the two double bonds, basically allowing the double bonds to be delocalized. The single bond hence has a "partial" double bond.
The hybridization is sp2 and with more s character the bond is shorter.

 

[folktale]

I'd say I & III.

Since it's a conjugated system, the pi bonds have to be delocalized. I is correct.

The increase in carbons (EDG) should increase the bond length as more electrons repel each other. II is wrong.

Since the single bond (middle bond) in 1,3butadiene technically does not contain a double bond, it has more s character than the 1st and 3rd carbons. III is correct.

 

[AZag13]

There is overlap between the two double bonds, basically allowing the double bonds to be delocalized. The single bond hence has a "partial" double bond.
The hybridization is sp2 and with more s character the bond is shorter.

The answer is D, with the explanation so kindly put by Jarudy.

 

[bajoneswadup]

Well its conjugated so that explains why l is correct. As for lll, not sure why that has more s character than ethane. If the question is talking about comparing the single bond to the double bond in 1,3-butadiene, then still not sure lol. All of the carbons are sp2 right?


Ohh nvm