Alkenes and Alkynes(E1 and E2 reactions) Flashcards
Alkenes
contains double bonds
relatively reactive, considered as a functional group
Alkenes: unsaturated are capable of adding….
adding hydrogens in the presence of a catalyst to form a saturated alkane
Elements of unsaturation: _______ the number of hydrogen atoms
decrease
Double bond equivalent DBE
(2C + 2 + N – H – Hal)/2
triple bonds counts as __ double bonds
2
Rotation about the double bond could potentially generate ________ isomer
conformational
( Compounds containing double bonds can exist as geometric isomers )
What is the elimination reaction?
loss of two atoms or groups from the substrate, usually with formation of a new pi bond (C=C)
Elimination of a proton and a halide ion is called dehydrohalogenation, and the product is an alkene
Unimolecular (E1, first-order)
(E1, first-order)
bimolecular
(E2, second-order)
E1 reaction - Rate-limiting transition state involves a single molecule (as in SN1)
STEPS
1) Unimolecular ionisation to give a carbocation (rate-limiting)
2) Deprotonation by a weak base (often the solvent) gives the alkene (fast)
E1 and Sn1 have the ____ order of reactivity on carbocation stability:
3»_space; 2» 1(no)
Good ionising solvent (alcohol or water)
Without a strong base → would force __ reaction
E2
Competition between E1 and SN1
Steps
1) Ionisation to form a carbocation
2) Basic attack by the solvent abstracts a proton to give an alkene (by the E1 mechanism)
3) Nucleophilic attack by the solvent on the carbocation (by the SN1 mechanism)
Rearrangement in E1 reaction 3 STEPS
1: Ionisation to form a carbocation
2: A hydride shift forms
a more stable carbocation (fast)
3: The weakly basic solvent removes either adjacent proton (fast)
What is the Zaitsev’s Rule?
most substituted alkene usually predominates
- tetrasubstituted
-tri”
-di”
-mono”
E2 reaction
Rate-limiting transition state involves two molecules (as in SN2)
SN2 mechanism is blocked because the tertiary alkyl halide is too hindered
tertiary will be slower
Reactivity of alkyl halides (substrate):
3° > 2° > 1°. Reflects the greater stability of highly substituted double bonds. Follow Zaitsev’s rule
Using bulky bases (B) can favour
E2 and SN2
large alkyl groups on a bulky base hinder its approach to attack a carbon atom
primary - StrongNuc/ base
SN2 reactions occur
primary - weak nuc/base
no reaction occurs
secondary - Strong Nuc/base
SN2 + E2
secondary - Weak Nuc/base
SN1 = E1 (slow)
teritary - strong nuc/base
E2
teritary - weak nuc/base
SN1 + E1
alkyl halides - strong nuc/ base
SN2 = E2
alkyl halides - weak nuc/ base
SN1 + E1
Methods for Synthesis of Alkenes (FGI)
6 methods in total
- Dehydrohalogenation of alkyl halides
(from alkyl halide to C=C) - Dehydration of alcohols
(from alcohol to C=C) - Dehydrogenation of alkanes
(from alkane to C=C) - Hofmann and Cope eliminations
- Reduction of alkynes
(from C≡C to C=C) - Witting reaction
Alkene Synthesis by dehydration of alcohols
Step 1: Protonation of the hydroxy group (fast equilibrium)
Step 2: Ionisation to a carbocation (slow; rate limiting)
Step 3: Deprotonation to give the alkene (fast)
Alkene double bond is a gateway functional group and transforms ____ bond to ______ bond
pi
sigma
Nucleophiles attack the carbocation forming a stable product:
which causes
Electrophilic Addition
Carbocation causes the compound to be +ive and therefore is an _______
electrophile
Electrophilic addition to alkenes
2 steps:
1) Attack of the pi bond on the electrophile forms a carbocation
2) Attack by a nucleophile gives the addition product
Markovnikov’s Rule is:
Addition of a proton acid to C=C
when adding a proton acid to C=C the product with the acid proton bonded to the carbon atom that already holds the….
greater number of hydrogen atoms
Positive charge on less substituted carbon. this means;
Less stable; not formed
Electrophilic addition to an alkene:
electrophile adds in such a way to generate the most stable intermediate (carbocation)
Anti-Markovnikov addition:
in some reactions the addition happen to the most substituted carbon
Addition of water: hydration of alkenes (FGI)
what are the 3 steps?
1) Protonation of the double bond forms a carbocation
2) Nucleophilic attack by water gives a protonated alcohol
3) Deprotonation gives the alcohol
Addition of halogens to alkenes (FGI)
2 steps
1) Electrophilic attack forms a halonium ion - the halogen is electrophilic > leaving group receives the lone pair > halonium ion, strong electrophile
2) The halide ion (nucleophile) opens the halonium ion, forming vicinal dihalides
water acts as a nucleophile = halohydrin
Markovnikov’s Rule - rule of thumb
attach the groups where there is the LEAST amount of hydrogens
alkenes reactions (FGI)
- Catalytic Hydrogenation: H2 added across the double bond
- Epoxidation: form a three-membered cyclic ether with new carbon–oxygen bonds
- Dihydroxylation: converting an alkene to a glycol
- Oxidative Cleavage: stronger conditions can further oxidise glycols to cleave the bond that was originally the double bond
- Oxymercuration–demercuration: another method for hydrating an alkene with Markovnikov orientation
- Hydroboration–oxidation: method for hydrating an alkene with anti-Markovnikov orientation
What is Ozonolysis:
type of weak oxidative cleavage where we cleave alkenes (double bonds) into either ketones, aldehydes or carboxylic acid using ozone
… its milder, and both ketones and aldehydes can be recovered
Electronic structure and acidity of Alkynes
Formed by sp hybridised carbons
Triple bond: 1) one sigma-bond and 2) two pi-bonds
1) 1 x sp – sp
2) 4 spare p orbitals
What kind of bases deprotonate terminal acetylenes to form carbanions called acetylide ions (or alkynide ions)
very strong bases
first step of synthesis of alkynes? (FGI)
1) Alkylation of acetylide ions
must be a primary alkyl halide; no bulky substituents or branches close to the reaction centre
IF back-side approach is HINDERED, acetylide acts as STRONG base: E2 mechanism
second step of synthesis of Alkynes? (FGI)
2) Addition of acetylide ions to carbonyl groups (C=O)
acetylide ion > carbonyl compound (aldehyde/ketone) > alkoxide ion (strong base) > an acetylenic alcohol
third step of synthesis of alkynes? (FGI)
3) Elimination reactions
reaction in extremely basic conditions (KOH-fused, 200C)
a) addition of water = addition to the triple bond
a. catalysed by HgSO4/H2SO4
Hydration of an alkyne initially gives a vinyl alcohol that cannot be isolated, because it quickly rearranges to a ketone
(acid-catalysed keto–enol tautomerism)
b) addition of water = addition to the triple bond
b. Hydroboration–oxidation
Hydration of an alkyne initially gives a vinyl alcohol that cannot be isolated, because it quickly rearranges to a aldehyde
(basic-catalysed keto–enol tautomerism)
