What is the structure of an alkene?
Pi bond - Overlap of 2 atomic orbitals
Sigma bond - Overlap of 2 sp2-hybridised orbitals
Contains a double bond (pi + sigma), and are thus more reactive than alkanes
What is the functional group of an alkene?
C=C
Alkene nomenclature
-ene instead of -ane e.g. propane => propene
The longest carbon chain containing the double bond is the parent chain
Start number closest to the double bond
The lower number indicates bond location
If it is a cycloalkene, start numbering at the double bond
Follows regular iupac rules for 2+ groups
Why do alkenes experience geometric isomerism?
Rotation around sigma bonds leads to isomers, this is due to double bonds being more rigid than single bonds.
What type of isomerism do alkenes experience?
cis-trans isomers
Cis = Similar groups on the same side of the double bond
Trans = Similar groups on opposite sides of the double bond
If either of the carbons have the same 2 groups attached to it, it is neither cis, nor trans
Must include this in the name (prefix cis- or trans-)
What if an alkene does not have cis-trans isomers? How do you distinguish between the forms it can have?
E-Z nomenclature
This is used when there are 4 different groups across the double bond carbons
What are the steps to determining E-Z nomenclature?
- Consider both carbons
- Assign priority to the groups similar to R/S
- If a group has a double/triple bond, then multiply the atomic number by the bond (e.g. C=O becomes 2C-2O)
- If the 1st priorities are on the same side = Z-isomer
On different sides = E-isomer
What are the property trends for alkenes?
Similar to alkanes
Higher molecular weight with less branching = higher boiling points
Slightly higher polarity than alkanes
Trans alkenes have no molecular dipole
Cis alkanes have higher boiling points and densities than trans alkanes
Conjugated alkenes are more stable
What is the stability trends for alkenes?
Less hydrogens bonded to the double bond carbons = higher stability
Trans isomers are more stable due to not experiencing steric strain
What is a conjugated double bond?
When multiple double bonds are present in a molecule, but are separated from each other by one single bond
What is an isolated double bond?
When multiple double bonds are present in a molecule, but are separated from each other by more than one single bond
How are alkenes synthesized?
Unimolar elimination (E1)
Bimolar elimination (E2)
Alcohol dehydration
Explain unimolar elimination
2 steps:
1. The polar C-X bond is broken to form a carbocation
2. The carbocation loses a H+ to form an alkene
This reaction takes place when a weak base is used
The carbocation can rearrange to form a more stable carbocation
E1 vs SN1 competition
Both can take place once a carbocation is formed, and in most cases both occur
Explain bimolecular elimination
1 step:
1. C-X bond cleavage, while simultaneous removal of H+ occurs
Concerted reaction
Happens in the presence of a strong base
Uses Zaitsev’s rule
Forms no intermediates, but can have a transition state
What stereochemistry is needed for E2 reactions?
Requires coplanar arrangement
H and X must be anti to each other
Without this arrangement (anti-coplanar) there exists too much steric strain for E2 to occur
E1 vs E2
- Base type
- Solvent type
- Substrate type
- Major product
- Stereochemistry needed
- Rearrangements
- E2 = Strong base, E1 = Weak base
- E2 = non-polar, E1 = Polar
- E1 requires a substrate capable of forming a stable carbocation, E2 doesn’t
- Follows Zaitsev’s rule, excepts when bulky bases are used for E2
- E2 = Anti-coplanar groups, E1 = None
- Only for E1 reactions
SN vs E reaction rules
SN1 vs E1: Weak bases & electrophiles
- 3° alkyl halides - Always SN1 and E1
- 2° alkyl halides - Always
- Higher temperature prefers E reactions
SN2 vs E2: Strong bases
- 3° halides - Never SN2
- 2° halides - always SN2 or E2
- 1° halides - Always both with weak base/acid
- Bulky bases prefer E2
- Good nucleophiles that are weak bases favour SN2
- Higher temp = more SN2
