Advices For Studying Organic Chemistry

1. The bond length of 1,3-butadiene:

CH 2 CH CH CH 2

1 234

1.34 Å 1.47 Å 1.34 Å

1) The C1―C2 bond and the C3―C4 bond are (within experimental error) the

same length as the C–C double bond of ethene.

2) The central bond of 1,3-butadiene (1.47 Å) is considerably shorter than the

single bond of ethane (1.54 Å).

i) All of the carbon atoms of 1,3-butadiene are sp^2 hybridized ⇒ the central bond

results from overlapping sp^2 orbitals.

ii) A σ bond that is sp^3 - sp^3 is longer ⇒ the central bond results from overlapping

sp^2 orbitals.

3) There is a steady decrease in bond length of C–C single bonds as the

hybridization state of the bonded atoms changes from sp^3 to sp.

Table13.1 Carbon-Carbon Single Bond Lengths and Hybridization State

Compound Hybridization State Bond Length (Å)

H 3 C―CH 3 sp^3 -sp^3 1.54

H 2 C=CH―CH 3 Sp^2 -sp^3 1.50

H 2 C=CH―CH=CH 2 Sp^2 -sp^2 1.47

HC≡C―CH 3 sp-sp^3 1.46

HC≡C―CH=CH 2 sp-sp^2 1.43

HC≡C―C≡CH sp-sp 1.37

13.7B CONFORMATIONS OF 1,3-BUTADIENE

1. There are two possible planar conformations of 1,3-butadiene:
   the s-cis and the s-trans conformations.

rotate about

C2−C3

23 2

3

s-cis Conformation of 1,3-butadiene s-trans Conformation of 1,3-butadiene