[Ciara]

Why does dichlorocarbene react with cycloocta-1,5-diene but not with the alkene tetrachloroethene?
 also inthis reaction why is the product cis-5,5,10,10-tetrachlorotricyclodecane and not the trans isomer???    Can somebody please help me i've been trying to figure this out for ages, thanks

[Arctic-Nation]

Two reasons for the selectivity exist. In the first place, dichlorocarbene exists as a singlet carbene with empty p orbitals, which makes it electrophilic. Tetrachloroethylene is electron-poor, which makes it a poor target for an electrophile. In the second place, singlet carbenes react in a concerted manner (which means in one step). The result is that when a singlet carbene approaches an ethylene head-on, two new bonds are formed simultaneously, leaving no time for isomerization.

Your cyclooctadiene is a special case, as the double bonds are locked into the cis conformation (the molecule is a ring), which would even when using a triplet carbene result in the formation of a double cis product.

[Ciara]

Thanks that make sense