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CHEM FOR BIOMED > Alkenes, Benzenes, SN1/SN2 and E1/E2 > Flashcards

Alkenes, Benzenes, SN1/SN2 and E1/E2 Flashcards

Lectures 14-20

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1
Q

General formula

A

CnH2n

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2
Q

Geometry of alkenes

A
  • contain C=C double bond

* Geometry of carbons is trigonal planar (bond angle of 120°)

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3
Q

Describe the E/Z nomenclature of alkenes

A
  1. assign priorities
  2. if the high priority groups are on the:
    SAME side - (Z)
    OPPOSITE sides - (E)

• sides are divided horizontally by the double bond

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4
Q

Describe the stability of different alkenes

A
  • The more substituted the alkene, the more stable it is.

* disubstituted is more stable than monosubstituted

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5
Q

Requirements for aromaticity

A
  • cyclic
  • conjugated (Alternating double and single bonds)
  • planar
  • Huckel’s rule: 4n + 2 ∏ electrons where n=integer
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6
Q

Disubstituted benzenes nomenclature

A

1,2 - Ortho
1,3 - meta
1,4 - para

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7
Q

pKa values and the strength of acid

A

Lower the pKa value, the stronger the acid

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8
Q

What are the two types of bond breaking?

A
  • homolytically: Each atom of bond gets one electron to form 2 radicals (odd electron species)
  • heterolytically: Atom of bond with higher electronegativity gets 2 electrons to form 2 even electron species
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9
Q

What are the two types of bond making?

A
  • Homogenic: 2 radicals donate one electron to form a bond

* heterogenic: two electrons donated from one species to another to form a bond

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10
Q

Nucleophile

A

• Electron rich molecules or ions that possess a non-bonding lone-pair of electrons (a.k.a Lewis Bases)

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11
Q

Electrophiles

A
  • Electron deficient molecules or ions

* usually lack a stable octet and can accept a pair of electron (a.k.a Lewis Acids)

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12
Q

How is electronegativity related to nucleophile/electrophiles?

A
  • delta + : less electronegative, electrophilic

* delta - : more electronegative therefore nucleophilic

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13
Q

Electron density maps

A
  • Illustrates the electron density of the regions in the molecule
  • red - high density = nucleophilic
  • Purple - low density = Electrophilic
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14
Q

Transition state

A

Highest energy state along the reaction coordinate

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15
Q

Polarity

A

Unsymmetrical distribution of electrons in a bond due to difference in electronegativities

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16
Q

Nucleophilic substitution reaction

A

Replacement of a leaving group by a nucleophile

• involves a leaving group (usually the molecule with higher electronegativity)

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17
Q

What are the 2 types of nucleophilic substitution reactions?

A

SN1 - stepwise (2 step reaction)

SN2 - concerted (simultaneous, one step reaction)

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18
Q

What are the factors which determine if reaction is SN1 or SN2?

A
  • nature of alkyl group
  • nature of leaving group
  • nature of nucleophile
19
Q

Rate of SN1 reaction

A

rate = k x [R-X]

Rate only depends on the concentration of the substrate

Rate: 3° > 2° > 1°
• Tertiary carbocation is most stable therefore fastest rate

20
Q

Describe an SN1 reaction

A
  1. Ionisation: leaving group leaves (Slow step)
  2. nucleophilic attack
  • involves a carbocation intermediate
  • End up with a racemic mixture
  • weak nucleophiles
21
Q

Rate of SN2 reaction

A

Rate = k x [R-X][Nu]

Rate depends on concentration of both the substrate and nucleophile

Rate: 1° > 2° > 3°
• Tertiary has most steric hindrance

22
Q

SN2 reaction

A
  • breaking and making of bond is simultaneous
  • Nucleophile approaches from a direction 180 deg away from leaving group
  • Walden inversion - only end up with one enantiomer if we start with one enantiomer
23
Q

What is an optically pure sample?

A

A sample that contains only one enantiomer

24
Q

SN1 or SN2 reaction for tertiary carbons?

A

SN1 preferred
• intermediate is very stable
• tertiary is too sterically hindered for SN2

25
Q

SN1 or SN2 reaction for primary carbons?

A

SN2 preferred
• sterically accessible
• not SN1 because resulting carbocations are unstable

26
Q

What is special about secondary substrates?

A

Can undergo either SN1 or SN2 reactions and it is often difficult to predict which is more likely

27
Q

Are weak bases better nucleophiles or leaving groups?

A
  • Weaker bases are usually better leaving groups
  • Stronger bases are better nucleophiles

Strong bases attack to displace the weaker base

28
Q

What is an elimination reaction?

A

The loss of an atom or group resulting in the formation of a multiple bond.

29
Q

What is the main difference between elimination and substitution reactions?

A
  • usually, basicity parallels nucleophilicity
  • for elimination, we require strong bases that are relatively weak nucleophiles

e.g. strong base can be a weak nucleophile due if it has high steric hindrance

30
Q

E1 reaction steps

A
  1. ionisation - leaving group leaves (Slow step) and forms a carbocation
  2. deprotonation - by losing a proton, carbocation becomes a more stable species
31
Q

E2 reaction

A

• breaking and making of bonds is simultaneous or concerted

32
Q

Describe the products of an elimination reaction

A

Elimination reactions can result in more than one product
• Major products: The ones that are ‘more substituted’ and hence more stable
• Minor product

disubstituted - double bond carbons attached to 2 carbons

33
Q

How does temperature influence the type of reaction that occurs?

A

• High temperatures - 100℃
Elimination reactions are more likely

• Low temperatures - 0℃
Substitution reactions more likely

34
Q

What reaction is mostly likely to occur if H2O is the nucleophile?

A
  • H2O is a weak nucleophile
  • therefore can only attack an unstable carbocation
  • carbocations are formed in 2 step reactions (SN1 or E1)
  • E1 for hot, and SN1 for cold
35
Q

What reaction is most likely to occur between a tertiary haloalkane and OH-?

A
  • OH is a strong base
  • so it is either, SN2 or E2
  • tertiary haloalkane - high steric hindrance for nucleophile to approach it, so SN2 cannot occur
36
Q

What reaction is most likely to occur with t-Bu as a nucleophile?

A
  • t-Bu is a strong base hence a strong nucleophile (SN2 or E2)
  • due to steric hindrance it cannot coordinate an SN2 reaction
  • therefore an E2 reaction occurs
37
Q

Is Br2 a nucleophile or electrophile?

A
  • When Br2 is approached by a nucleophile, it becomes polarised
  • one side is delta + and another is delta -
  • therefore it can react with a nucleophile, hence is considered as an electrophile
38
Q

What is Zaitsev’s rule?

A

In the elimination fo H-X from an alkyl halide, the more highly substituted, and less sterically crowded alkene product predominated (major).

• In practise, this means there will be as few H atoms as possible bonded to C=C bond.

39
Q

How to determine whether substitutions or elimination will occur?

A

Bulky nucleophile - elimination
• A bulky attacking nucleophile suffers more steric hindrance attacking an electrophile in substitution than elimination

Secondary, tertiary alkyl halide more likely to undergo elimination than primary
• In a substitution reaction, electrophilic carbon atom being attacked is more sterically hindered in secondary or tertiary - so elimination occurs

40
Q

What are good leaving groups?

A
  • ones that can be displaced easily

* strong bases are bad leaving groups (i.e. OH-)

41
Q

When will secondary haloalkanes undergo SN1 or SN2 reactions?

A

SN1 favoured in
• protic solutions - ones that contain a H proton that can be easily displaced/donated (ie. OH, NH3, H2O)
• with weak nucleophiles

SN2 favoured in
• aprotic solutions - cannot donate hydrogen
• with strong nucleophiles

42
Q

Why are polar protic solvents favoured by SN 1 reactions?

A
  • polar protic solvents have hydrogen atoms connected to electronegative atoms, and can be easily donated
  • in sn1 reactions a carbocation intermediate is formed
  • leaving group becomes a anion
  • the H atom in the polar protic molecules can stabilise both the carbocation and anion (leaving group)
43
Q

Why are polar aprotic solvents favoured by SN2 reactions?

A

• SN2 reactions are one-step reactions involving a strong nucleophile
• the polar aprotic solvents is able to stabilise the cation formed by the nucleophile
i.e. in nucleophile NaOH, Na+ will be stabilised to increase the ability of OH to attack

CHEM FOR BIOMED (14 decks)

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