Alkene Structure and Naming

Alkene

A hydrocarbon with at least one C=C double bond. Formula CₙH₂ₙ

Unsaturated hydrocarbon

Has fewer H than the fully single bonded alkane because it has double or triple bonds

Hybridization and shape of alkene carbons

sp² and trigonal planar with about 120° angles

What a C=C double bond is made of

One sigma bond plus one pi bond

Pi bond

Formed by side by side overlap of unhybridized p orbitals that must stay parallel

Why a C=C can't rotate

Rotating would twist the p orbitals out of overlap and break the pi bond

Naming change from alkane to alkene

The -an- becomes -en- like hept-1-ene

How to number an alkene chain

Give the C=C the lowest number unless a higher priority suffix like -ol or -one is present

Naming a molecule with two C=C

Use -adiene with both numbers like octa-2‚6-diene

Numbering a ring alkene

The two C=C carbons are C1 and C2 then give substituents the lowest numbers

Z configuration

Higher priority groups on the same side (zusammen means together)

E configuration

Higher priority groups on opposite sides (entgegen means apart)

CIP priority rules

Higher atomic number wins and if tied go to the first point of difference and double bonds count as two of that atom

When an alkene has no E or Z (N/A)

When either carbon of the double bond has two identical groups

Which wins: ethyl or methyl

Ethyl because it wins at the second atom (first point of difference)

Which wins: CH₂OH or isopropyl

CH₂OH because O beats C at the first point of difference

Cis/trans vs E/Z

They usually match but not always. E/Z is based on priority so always use priority

Where E or Z goes in the name

In parentheses at the very start like (E)-2-chloropent-2-ene