strength of C=C and C-C

The first statement is false.

A C=C double bond is made of 4 bonding electrons, while a C-C single bond is made of 2 bonding electrons. Actually, the repulsion between 4 bonding electrons must be greater than that between 2 bonding electrons. Therefore, the bonding electrons in a C=C double bond must repel each other much more strongly than those in a C-C single bond.

However, the nuclear attractions between the 2 atomic nuclei and the 4 bonding electrons in a C=C double bond is greater than those between the 2 atomic nuclei and the 2 bonding electrons in a C-C single bond. The factor of the increase in nuclear attractions of bonding electrons outweighs the factor of the increase in repulsions between bonding electrons. Therefore, a C=C double bond is stronger than a C-C single bond, but we cannot determine how much stronger.


The second statement is true.

A C=C double bond is made of a σ bond a π bond, while a C-C single bond is made of a σ bond. A π bond is much weaker than a σ bond, because the π bond is just a side overlapping and the effect of side overlapping is low. Therefore, the bond dissociation enthalpy of a C=C double bond is higher than that of a C-C single bond, but not twice (actually lower than twice).

2009-09-05 00:55:46 補充：
yaumango11 has only attempted to explain the angles between the electron pairs based on the repulsion between the bond pair and lone pairs, but not the repulsion between the electron pairs in the C=C double bond.

你個reason 1 其實可有可無
reason 2先係真正主要原因.

第1點 , 影響比較微 , 因為C=C鍵的C是sp2雜化 , 有 1 sigma bond and 1 pi bond , 抗拒電子間斥力的最有效排列方法是三角平面。