6.1: Cahn-Ingold-Prelog Convention
- Page ID
- 321423
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Alkenes are hydrocarbons that contain a C=C double bond. We name them similarly to alkanes, but use the suffix –ene instead of –ane. We normally indicate the position of the double bond with a number. For example, 2-butene (or but-2-ene) and 1-butene (or but-1-ene) have different structures.
When substituents are placed along the chain, the priority rules much be followed. Alkenes are higher priority than halides, but lower priority than alcohols, for example. So:
There are three other groups that contain a C=C double bond which have common names:
E/Z nomenclature of alkenes
The Cahn-Ingold-Prelog convention dictates that we assign priority to the various groups attached to the alkene in order to distinguish between isomers. We use the prefixes E- (for entgegen, German for “opposite”) to indicate that the two highest priority groups lie on opposite sides of the alkene, or Z- (for zusammen, German for “same”) to indicate that the two highest priority groups lie on the same side. This E/Z system of nomenclature assigns priority based on atomic number:
- atoms that are directly attached to the alkene are assigned priority based on decreasing atomic number (highest atomic number = highest priority)
- if two groups have the same atomic number, consider atomic mass (higher atomic mass = higher priority)
- if two groups have the same atomic mass AND number, move along the chain to a different atom until there is a difference in priority
- if two groups have the same atomic number AND atomic mass AND lie in the same position along the chain (say, -CH2OH vs. –CH=O), but one of them is doubly bonded (such as C=O), then count that atom twice (as if it were attached to two oxygen atoms. But beware that this is not additive!)
- if two highest priority groups are on the same side = Z, opposite sides = E. Note that we do not use the terms cis and trans when naming alkenes, only when indicating the relative position of groups.
We call the Z and E isomers of the same compound geometric isomers, which is a class of stereoisomers known as diastereomers. These isomers do not have the same physical properties because they have different energies. In fact, E-but-2-ene is 0.7 kcal/mol more stable that Z-but-2-ene. There is a van der Waals interaction when two groups come into close contact, such as the cis methyl groups. This is steric strain that drives up the energy of the cis isomer.
