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MCAT - Organic Chemistry > Class 3 > Flashcards

Class 3 Flashcards

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1
Q

What kind of bond is made/broken in a substitution reaction?

A

One sigma bond exchanged for another sigma bond.

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2
Q

What are the differences between an SN1 and SN2 reaction?

A

SN1:

Stereochemistry is not necessarily retained
Substitution occurs at primary center not tertiary

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3
Q

How do nucleophiles attack and what does this cause?

A

“Concerted” backside attack. Causes the molecule to “flatten out” as one bond is forming while the other is breaking. This is a high energy “pentavalent” transition state.

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4
Q

Do SN1 or SN2 reactions require a stronger nucleophile?

A

SN2

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5
Q

In regards to carbon substitution, what is preferred for SN2 reactions?

A

CH3

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6
Q

What is the rate law for SN2 reactions?

A

Rate = k[E+][Nu-]

SN2

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7
Q

What solvent is used for SN2 reactions?

A

Polar, aprotic (e.g. acetone, DMSO, DMF)

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8
Q

How can you turn OH into a good leaving group?

A
  1. Acid catalyze to OH2+
    HCl is a popular acid for this
  2. Replace with TslCl
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9
Q

How many steps are in an SN1 reaction?

A

2!

  1. Carbocation formation; this is the slow, rate-limiting step
  2. Nucleophilic attack
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10
Q

How many steps are in a SN2 reaction?

A

1! Concerted LG leaving/Nu attack

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11
Q

How does a SN1 nucleophile attack?

A

Can attack on EITHER side of the p orbital (carbocation) so we get BOTH stereoisomers in the product

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12
Q

In regards to carbon substitution, what is preferred for SN1 reactions?

A

3 -> 2 ->->->1 ->->-> CH3

Really only tertiary or secondary carbons will ever successfully form a carbocation intermediate. Poorly subtsituted carbocats. won’t be inductively stabilized enough

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13
Q

What is the rate law for an SN1 reaction?

A

Rate = k[E+]

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14
Q

What kind of nucleophile is desired for an SN1 reaction?

A

A relatively weak nucleophile is preferred

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15
Q

What solvent is used for SN1 reactions?

A

Polar, protic (e.g. H2O, ROH)

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16
Q

What are the two ways in which a carbonyl is reactive?

A
  1. The carbonyl carbon is electrophilic

2. The alpha carbon is acidic

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17
Q

What is tautomerization?

A

Rapid equilibration between structural isomers.

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18
Q

If the MCAT asks for deuterium exchange using D2O, what are they asking you to find?

A

How many acidic hydrogens there are.

Think: hydrogens attached to alpha carbons

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19
Q

What bonds are made/broken in a nucleophilic addition reaction?

A

1 pi bond becomes 2 sigma bonds.

20
Q

What precursor step do you need to do a nucleophilic addition with a weak nucleophile?

A

First acid catalyze carbonyl to an O-H bond, then nucleophile attacks

21
Q

Aldehyde and ketone reactants yield ______ and _______ _________, respectively. Via what type of reaction?

A

Primary and secondary alcohols.

Via reduction!

22
Q

What are common reducing agents of carbonyls?

A
  1. NaBH4 or LiAlH4

2. Acid work up (H3O+)

23
Q

How do you form a Grignard Reagent?

A

CH3Br + Mg —–> (ether) Ch3MgBr

24
Q

What is a Grignard agent useful for?

A

Forming a new carbon-carbon bond using a carbonyl. The carbonyl will then become an OH in the acid work up step.

25
Q

If a nucleophile is on a more EN atom, is it a stronger or weaker nucleophile?

A

Weaker/worse cause that EN atom really likes to draw in and hold onto its electrons.

26
Q

What is an acetal and hemiacetal?

A

Acetal is a carbon bonded to OR and OR

Hemiacetal is a carbon bonded to OR and OH

27
Q

How do you form a hemiacetal and acetal?

A

With a carbonyl:

  1. H+ and R’OH

Hemiacetal, do this once. Acetal, do this twice.

28
Q

How is an imine formed?

A

Forms when a primary amine (RNH2) is the nucleophile (attacks a carbonyl)

29
Q

How is an enamine formed?

A

Forms when a secondary amine (R2NH2) is the nucleophile (attacks a carbonyl)

30
Q

Do imines and enamines need to be acid catalyzed?

A

Yes

31
Q

What additional step is needed in forming an enamine?

A

Carbon must be deprotonated using water -> allows electrons from nitrogen pi bond to push up and alleviate positive charge on nitrogen.

32
Q

Can an enamine be nucleophilic?

A

Yes. The alpha carbon is acidic like an enolate

33
Q

What do you use to pull off the acidic H on an enolate, imine, enamine?

A

base

34
Q

What is the product of an aldol reaction?

A

beta-hydroxy carbonyl or an

alpha,beta-unsaturated carbonyl

35
Q

What is the difference between a kinetic enolate and a thermodynamic enolate?

A

Kinetic enolate is less substituted, less stable, but forms faster and at lower temperatures. Usually can be formed using a bulkier base.

Thermodynamic enolates are more substituted, more stable, and forms at high temperatures with small bases.

36
Q

Are carboxylic acids highly polar and form many H bonds? What is the consequence of this?

A

Yes. Therefore have a high BP

37
Q

How can you reduce a carboxylic acid to a primary alcohol?

A
  1. LiAlH4

2. H3O+

38
Q

How does a nucleophilic addition-elimination reaction work?

A
  1. Nucleophile attacks a carbonyl thats also bound to an electronegative species, pushing electrons onto the oxygen
  2. Electrons on oxygen push back down, kicking out the EN species.
39
Q

How can your form an ester?

A

Carboxylic acid + primary alcohol in acidic conditions

40
Q

What is saponification?

A

Using an ester and a strong base (like OH-) will form a carboxylate anion, kicking off OEt. The carboxylate anion can be made into a carboxylic acid with an additional acid work up

41
Q

What are some major carboxylic acid derivatives and what are there relative reactivities? What can be made into what?

A
  1. Acid chloride
  2. Acid Anhydride
  3. Ester
  4. Amide

(in order from best to worst LG/reactivity)
Anything in the above list can be made from anything below it.

42
Q

What makes a good leaving group?

A

A good leaving group has more stability in solution

43
Q

What does a transesterification do?

A

Swaps out R groups in an ester (–OR)

44
Q

Can you make an amide directly from a carboxylic acid and an amine? Why?

A

No because a proton transfer will occur first. The solution is to treat an amine with some other carboxylic acid derivative.

45
Q

How can you make a bad electrophile into a better one?

A

Protonate it to give it a + charge

OR make the nucleophile a stronger nucleophile by deprotonating it.

MCAT - Organic Chemistry (5 decks)

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