1 Alkenes and Alkynes
Alkenes, sometimes referred to as olefins, are hydrocarbons that contain one or more carbon-carbon double bonds, and alkynes are hydrocarbons that contain one or more carbon-carbon triple bonds. They are both unsaturated hydrocarbons because they have fewer hydrogen atoms than the corresponding alkanes.
Degree of Unsaturation
Alkanes containing only single bonds and no rings are saturated hydrocarbons. The general formula for these alkanes is CnH2n+2. In contrast, an alkene containing one double bond has the formula CnH2n—it has two fewer hydrogen atoms. An alkyne containing one triple bond has the formula CnH2n-2, with four fewer hydrogen atoms.
This reduction in number of hydrogen atoms is referred to as degree of unsaturation, and one degree of unsaturation, or an unsaturation number of 1, corresponds to having two hydrogen atoms fewer. The unsaturation number (U) can be calculated using the formula
where C is the number of carbon atoms, N is the number of nitrogen atoms, X is the number of halogen atoms (F, Cl, Br, I), and H is the number of hydrogen atoms in the molecule.
Exercise 1: Determine the unsaturation number
Exercise 2: Isomers
For a challenge, determine the unsaturation number of C3H2, and draw all the possible constitutional isomers of C3H10. (Of these species, only one has not been detected in interstellar space. Of the 190+ molecules that have been detected, many are highly unsaturated carbon-rich species like C3H2.)
Answer the question (write and draw) in your notebook, then left-click here.
C3H2 has an unsaturation number of 3.
Carbon-Carbon Double Bond
A C=C bond consists of a σ bond and a π bond. The carbon atoms involved in the double bond are sp2 hybridized with a trigonal planar geometry.
For example, in ethene, H2C=CH2, each sp2 hybridized C atom has three σ bonds in a plane: two of the sp2 hybrid orbitals go toward forming two C–H σ bonds and the third sp2 hybrid orbital forms a C-C σ bond. The C-C π bond is formed from the overlap of the unhybridized 2p atomic orbital on each carbon atom, perpendicular to the plane of the sp2 hybrid orbitals.
It is the presence of this C-C π bond that gives alkenes their particular reactivity. Generally, the two π electrons are at a higher energy than σ electrons, making them more energetically accessible. In other words, alkenes can act as nucleophiles, where the two π electrons go toward reacting with an electrophile.
Carbon-Carbon Triple Bond
A C≡C bond consists of a σ bond and two π bonds. The carbon atoms involved in the triple bond are sp hybridized with a linear geometry.
For example, in acetylene, H−C≡C−H, each carbon atom is sp hybridized with two unhybridized 2p atomic orbitals. One sp hybrid orbital from each C atom overlaps to form a C-C σ bond, the other sp hybrid orbital forms a C-H σ bond with a hydrogen atom. The unhybridized 2p atomic orbitals overlap to form two perpendicular C-C π bonds. The two carbon atoms of acetylene are thus bound together by one σ bond and two π bonds, giving a triple bond.
Because alkyne reactions typically involve their π electrons, the reactivity of alkynes is quite similar to that of alkenes.