Alkenes and Alkynes reactions Flashcards
Double bond equivalent
- All double bonds
- All the cycles
- Double bond equal to 2
Elements of unsaturation
- Unsaturated. Capable of adding hydrogens in the presence of a catalyst to form a saturated alkane
Configurational Isomer
- Rotation about the double bond could potentially generate conformational isomer
- This cannot happen. Rotation would break the (strong) double bond between the carbon atoms
Cis trans
- Only applies when both ends of the double bond have two
- Different groups attached, but groups at one end of double bond are identical to those at the other
Elimination reaction
- Loss of two atoms or groups from the substrate, usually with formation of a new pi bond
- Carbon carbon double bond proton and a halide ion is called dehydrohalogenation, and the product is an alkene
- substitutions (SN1 and SN2) and both unimolecular or biomolecular
Substitution
- Sn1 and Sn2 substitution unimolecular and bimolecular
E1 reaction
- Unimolecular ionisation to give a carbocation instead of collision of 2 - curly arrow from carbon with halide to halide
- Deprotonation by a weak base
- Alcohol and water are good ionising solvents
- Without strong base force E2
Alkenes
- Hydrocarbons with carbon–carbon double bonds, sometimes called olefins
- relatively reactive, considered as a functional group
Alkenes 2 sp2 hybridised carbons, trigonal planar geometry
E/Z nomenclature
- Must be used when we have completely different groups at the two ends of the double bond check priority of groups
- E is on the opposite and z is on the same side
Competition between E1 and SN1
- Ionisation to form a carbocation carbon curly arrow to halide
- Removal of halide forms positve cation
- Nucleophilic attack by the solvent on the carbocation
Rearrangement in E1 reaction
- E1 may be accompanied by rearrangements such as hydride shifts and alkyl shifts
- Ionisation to form a carbocation
- A hydride shift forms a more stable carbocation
- The weakly basic solvent removes either adjacent proton
Zaitsev’s Rule
- In elimination reactions, the most substituted alkene usually predominates
- The more methyl groups takes priority most substituted carbon
Rearrangement in E1 and Zaitsev’s Rule
- In ethanol 3 elimination products are formed
- This secondary carbocation can lose a proton to give an unrearranged alkene
E2 reaction
- Rate-limiting transition state involves two molecules
- can be a strong base as well as a strong nucleophile, but react as base
- Mechanism is blocked because the tertiary alkyl halide is too hindered for Sn2
Reaction of alkyl halides E2
3° > 2° > 1°. Reflects the greater
stability of highly substituted double bonds follows Zaitsev rule - most substituted not favoured
- Having bulky base favours E2 over Sn2
Strong
Predicting product from base
Alkyl halide with a strong base form Sn2 and E2
Weak
Predicting product from base
Alkyl halide with a weak base form Sn1 and E1
Primary alkyl halide with strong base
- Reactants of typical E2 reaction
Secondary alkyl halide with strong base
- Can be E2 or Sn2
Secondary alkyl halide and weak base
- SN1 and E1 slow
Tertiary alkyl halide and strong base
- E2 reaction only able to occour due to steric hinderance
Tertiary alkyl halide and weak base
- E1 and E2 reactions
Hydration reaction
- Add strong acid to the alcohol
- Protonation of the alcohol forming H2O
- Now good leaving group ionisation to form carbon cation
- final hydrogen is then extracted by water
Alkene reactions
- Removal of the pi bond
- Strong electrophile pulls the electron away then carboncation is attacked by a nucleophile through electrophilic addition
Dehydration reaction
- Attack of pi bond on electrophile
- Attack you nucleophile gives additional product
Markonikov rule
- Addition of proton acid to carbon atom that holds the greatest number of hydrogen atoms
- More stable intermediate carbocation
Alkene to alcohol
- Protonation of double bond forms carbon cation
- Nucleophilic attack of water gives protonated alcohol
- Deprotonation of alcohol
Addition of halogen to alkene
- Halogen molecule is an electrophile
- Halide ion open up the halonium ion forms vicinal dihalides
Hydrogenation alkene reagent
- Ni & Pt and H2
Dihydroxylation of alkene
- Addition of Potassium permanganate for 2 alcohol
Oximercutration
Use the mercuary reagent hydrates the alkene by
Oximercutration
Use the mercuary reagent hydrates the alkene by
Hydroboration–oxidation
- hydrating an alkene with anti-Markovnikov orientation
- Reagent Boron BH3 THF
Alkyne
- Hydrocarbons with triple bond acetylenes
- 2 elements of unsaturation double bond equivelent
- 1sp bond and 2 pi bonds
Reactions with Alkynes
- Terminal alkynes more acidic proton that can be pulled by strong base
- Deprotonation of terminal acetylenes form carbon cations called acetylide ions
- Substitute acetylene via SN2 reaction (Must not be bulky so primary alkyl halide)
Addition to carbonyl group
- Oxygen is more electronegative compared carbon there carbon is electrophilic
- Aceylide ion attacks the carbonyl forming an alkoxide ion which is a strong base
- Addition of diluted acid H3O arrow from O to H3O forms an alcohol
Reduction of alkynes
- Platinum reagent used almost imposible to just reduce to only alkene so turns to alkane
