Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

PMP-102 > Alkenes and Alkynes(E1 and E2 reactions) > Flashcards

Alkenes and Alkynes(E1 and E2 reactions) Flashcards

1
Q

Alkenes

A

contains double bonds
relatively reactive, considered as a functional group

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Alkenes: unsaturated are capable of adding….

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Elements of unsaturation: _______ the number of hydrogen atoms

A

decrease

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Double bond equivalent DBE

A

(2C + 2 + N – H – Hal)/2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

triple bonds counts as __ double bonds

A

2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Rotation about the double bond could potentially generate ________ isomer

A

conformational

( Compounds containing double bonds can exist as geometric isomers )

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is the elimination reaction?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Unimolecular (E1, first-order)

A

(E1, first-order)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

bimolecular

A

(E2, second-order)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

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

A

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

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

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

A

3&raquo_space; 2» 1(no)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

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

A

E2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Competition between E1 and SN1
Steps

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Rearrangement in E1 reaction 3 STEPS

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the Zaitsev’s Rule?

A

most substituted alkene usually predominates

  • tetrasubstituted
    -tri”
    -di”
    -mono”
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

E2 reaction

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

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

A

tertiary will be slower

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Reactivity of alkyl halides (substrate):

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Using bulky bases (B) can favour

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

primary - StrongNuc/ base

A

SN2 reactions occur

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

primary - weak nuc/base

A

no reaction occurs

22
Q

secondary - Strong Nuc/base

A

SN2 + E2

23
Q

secondary - Weak Nuc/base

A

SN1 = E1 (slow)

24
Q

teritary - strong nuc/base

A

E2

25
Q

teritary - weak nuc/base

A

SN1 + E1

26
Q

alkyl halides - strong nuc/ base

A

SN2 = E2

27
Q

alkyl halides - weak nuc/ base

A

SN1 + E1

28
Q

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

A
  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
29
Q

Alkene Synthesis by dehydration of alcohols

A

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)

30
Q

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

A

pi
sigma

31
Q

Nucleophiles attack the carbocation forming a stable product:
which causes

A

Electrophilic Addition

32
Q

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

A

electrophile

33
Q

Electrophilic addition to alkenes
2 steps:

A

1) Attack of the pi bond on the electrophile forms a carbocation

2) Attack by a nucleophile gives the addition product

34
Q

Markovnikov’s Rule is:

A

Addition of a proton acid to C=C

35
Q

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

A

greater number of hydrogen atoms

36
Q

Positive charge on less substituted carbon. this means;

A

Less stable; not formed

37
Q

Electrophilic addition to an alkene:

A

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

38
Q

Anti-Markovnikov addition:

A

in some reactions the addition happen to the most substituted carbon

39
Q

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

A

1) Protonation of the double bond forms a carbocation

2) Nucleophilic attack by water gives a protonated alcohol

3) Deprotonation gives the alcohol

40
Q

Addition of halogens to alkenes (FGI)
2 steps

A

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

41
Q

Markovnikov’s Rule - rule of thumb

A

attach the groups where there is the LEAST amount of hydrogens

42
Q

alkenes reactions (FGI)

A
  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
43
Q

What is Ozonolysis:

A

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

44
Q

Electronic structure and acidity of Alkynes

A

Formed by sp hybridised carbons

45
Q

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

A

1) 1 x sp – sp
2) 4 spare p orbitals

46
Q

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

A

very strong bases

47
Q

first step of synthesis of alkynes? (FGI)

A

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

48
Q

second step of synthesis of Alkynes? (FGI)

A

2) Addition of acetylide ions to carbonyl groups (C=O)

acetylide ion > carbonyl compound (aldehyde/ketone) > alkoxide ion (strong base) > an acetylenic alcohol

49
Q

third step of synthesis of alkynes? (FGI)

A

3) Elimination reactions

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

50
Q

a) addition of water = addition to the triple bond

A

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)

51
Q

b) addition of water = addition to the triple bond

A

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)

PMP-102 (49 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions