Alkenes and Alkynes(E1 and E2 reactions)

Question 1

Alkenes

Answer 1

contains double bonds
relatively reactive, considered as a functional group

Question 2

Alkenes: unsaturated are capable of adding….

Answer 2

adding hydrogens in the presence of a catalyst to form a saturated alkane

Question 3

Elements of unsaturation: _______ the number of hydrogen atoms

Answer 3

decrease

Question 4

Double bond equivalent DBE

Answer 4

(2C + 2 + N – H – Hal)/2

Question 5

triple bonds counts as __ double bonds

Answer 5

2

Question 6

Rotation about the double bond could potentially generate ________ isomer

Answer 6

conformational

( Compounds containing double bonds can exist as geometric isomers )

Question 7

What is the elimination reaction?

Answer 7

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

Question 8

Unimolecular (E1, first-order)

Answer 8

(E1, first-order)

Question 9

bimolecular

Answer 9

(E2, second-order)

Question 10

E1 reaction - Rate-limiting transition state involves a single molecule (as in SN1)
STEPS

Answer 10

1) Unimolecular ionisation to give a carbocation (rate-limiting)

2) Deprotonation by a weak base (often the solvent) gives the alkene (fast)

Question 11

E1 and Sn1 have the ____ order of reactivity on carbocation stability:

Answer 11

3&raquo_space; 2» 1(no)

Question 12

Good ionising solvent (alcohol or water)
Without a strong base → would force __ reaction

Answer 12

E2

Question 13

Competition between E1 and SN1
Steps

Answer 13

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)

Question 14

Rearrangement in E1 reaction 3 STEPS

Answer 14

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)

Question 15

What is the Zaitsev’s Rule?

Answer 15

most substituted alkene usually predominates

• tetrasubstituted
  -tri”
  -di”
  -mono”

Question 16

E2 reaction

Answer 16

Rate-limiting transition state involves two molecules (as in SN2)

Question 17

SN2 mechanism is blocked because the tertiary alkyl halide is too hindered

Answer 17

tertiary will be slower

Question 18

Reactivity of alkyl halides (substrate):

Answer 18

3° > 2° > 1°. Reflects the greater stability of highly substituted double bonds. Follow Zaitsev’s rule

Question 19

Using bulky bases (B) can favour

Answer 19

E2 and SN2
large alkyl groups on a bulky base hinder its approach to attack a carbon atom

Question 20

primary - StrongNuc/ base

Answer 20

SN2 reactions occur

Question 21

primary - weak nuc/base

Answer 21

no reaction occurs

Question 22

secondary - Strong Nuc/base

Answer 22

SN2 + E2

Question 23

secondary - Weak Nuc/base

Answer 23

SN1 = E1 (slow)

Question 24

teritary - strong nuc/base

Answer 24

E2

Question 25

teritary - weak nuc/base

Answer 25

SN1 + E1

Question 26

alkyl halides - strong nuc/ base

Answer 26

SN2 = E2

Question 27

alkyl halides - weak nuc/ base

Answer 27

SN1 + E1

Question 28

Methods for Synthesis of Alkenes (FGI) 6 methods in total

Answer 28

1. Dehydrohalogenation of alkyl halides (from alkyl halide to C=C) 2. Dehydration of alcohols (from alcohol to C=C) 3. Dehydrogenation of alkanes (from alkane to C=C) 4. Hofmann and Cope eliminations 5. Reduction of alkynes (from C≡C to C=C) 6. Witting reaction

Question 29

Alkene Synthesis by dehydration of alcohols

Answer 29

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)

Question 30

Alkene double bond is a gateway functional group and transforms ____ bond to ______ bond

Answer 30

pi sigma

Question 31

Nucleophiles attack the carbocation forming a stable product: which causes

Answer 31

Electrophilic Addition

Question 32

Carbocation causes the compound to be +ive and therefore is an _______

Answer 32

electrophile

Question 33

Electrophilic addition to alkenes 2 steps:

Answer 33

1) Attack of the pi bond on the electrophile forms a carbocation 2) Attack by a nucleophile gives the addition product

Question 34

Markovnikov’s Rule is:

Answer 34

Addition of a proton acid to C=C

Question 35

when adding a proton acid to C=C the product with the acid proton bonded to the carbon atom that already holds the....

Answer 35

greater number of hydrogen atoms

Question 36

Positive charge on less substituted carbon. this means;

Answer 36

Less stable; not formed

Question 37

Electrophilic addition to an alkene:

Answer 37

electrophile adds in such a way to generate the most stable intermediate (carbocation)

Question 38

Anti-Markovnikov addition:

Answer 38

in some reactions the addition happen to the most substituted carbon

Question 39

Addition of water: hydration of alkenes (FGI) what are the 3 steps?

Answer 39

1) Protonation of the double bond forms a carbocation 2) Nucleophilic attack by water gives a protonated alcohol 3) Deprotonation gives the alcohol

Question 40

Addition of halogens to alkenes (FGI) 2 steps

Answer 40

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

Question 41

Markovnikov’s Rule - rule of thumb

Answer 41

attach the groups where there is the LEAST amount of hydrogens

Question 42

alkenes reactions (FGI)

Answer 42

1. Catalytic Hydrogenation: H2 added across the double bond 2. Epoxidation: form a three-membered cyclic ether with new carbon–oxygen bonds 3. Dihydroxylation: converting an alkene to a glycol 4. Oxidative Cleavage: stronger conditions can further oxidise glycols to cleave the bond that was originally the double bond 5. Oxymercuration–demercuration: another method for hydrating an alkene with Markovnikov orientation 6. Hydroboration–oxidation: method for hydrating an alkene with anti-Markovnikov orientation

Question 43

What is Ozonolysis:

Answer 43

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

Question 44

Electronic structure and acidity of Alkynes

Answer 44

Formed by sp hybridised carbons

Question 45

Triple bond: 1) one sigma-bond and 2) two pi-bonds

Answer 45

1) 1 x sp – sp 2) 4 spare p orbitals

Question 46

What kind of bases deprotonate terminal acetylenes to form carbanions called acetylide ions (or alkynide ions)

Answer 46

very strong bases

Question 47

first step of synthesis of alkynes? (FGI)

Answer 47

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

Question 48

second step of synthesis of Alkynes? (FGI)

Answer 48

2) Addition of acetylide ions to carbonyl groups (C=O) acetylide ion > carbonyl compound (aldehyde/ketone) > alkoxide ion (strong base) > an acetylenic alcohol

Question 49

third step of synthesis of alkynes? (FGI)

Answer 49

3) Elimination reactions reaction in extremely basic conditions (KOH-fused, 200C)

Question 50

a) addition of water = addition to the triple bond

Answer 50

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)

Question 51

b) addition of water = addition to the triple bond

Answer 51

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)