4.1.3 Alkenes GapFill

Like alkanes, alkenes are hydrocarbons. However, alkenes contain at least one carbon−carbon double bond and, hence, do not contain as many hydrogen atoms as the corresponding alkane; they are said to be  saturatedchiralunsaturatedachiral. The general formula for a non-cyclic alkene with n carbon atoms and one double bond is   CnH2n+2CnH2n+1CnH2nCnHn+1.

Carbons involved in a double bond are found in a trigonal planar environment, with bond angles around  109.5º90º120º180º. Carbon−carbon double bonds consist of one σ(sigma) bond, formed by end-to-end overlap of p orbitals, and one   θ(theta)π(pi)μ(mu)δ(delta) bond, formed by side-on overlap of p orbitals. This second component prevents rotation about the double bond, as it would disrupt the side-on overlap of orbitals. This leads to  P/ZP/XE/XE/Z isomerism.

Carbon−carbon double bonds are electron-rich, and a common class of reactions for alkenes is  electrophilic eliminationelectrophilic substitutionnucleophilic eliminationelectrophilic addition. In fact, a reaction of this type is used to test for unsaturation in organic molecules: a sample is reacted with   chlorinefluorinebromineiodine water, which changes from   greenpurpleorangeblue to colourless in the presence of a double bond. These reactions involve the formation of an intermediate with a positively charged carbon atom called a   protocarbonzwitterioncarbanioncarbocation.

Alkenes can also form   subtractionadditioncondensationdisplacement polymers.