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MCAT chem > Carbonyl Chemistry > Flashcards

Carbonyl Chemistry Flashcards

1
Q

Explain the stereochemistry of an sn2 rxn

A

backside attack leads to inversion of stereo-chem

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2
Q

what kind of solvent does sn2 rxns prefer? list examples.

A

polar aprotic solvents (non-H-bonding)

e.g. DMSO, DMF, and acetone

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3
Q

what substrates and nucleophiles are preferred by sn2 rxns?

A

since backside attack occurs, we need reduced steric hindrance –> primary substrates are better

to reduce hindrance, we want small (non-bulky) and strong nucleophiles (e.g. basic ones)

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4
Q

explain the stereochemistry of sn1 rxns

A

the first step of sn1 creates a planar carbocation. Thus, the nucleophile can attack from top or bottom creating a 50/50 racemate

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5
Q

what kind of solvent does sn1 rxns prefer? list examples.

A

polar protic solvents which stabilize the carbocation. they can also perform solvolysis in which the solvent acts as the nucleophile

e.g. H2O, NH3, alcohols

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6
Q

what are the most stable substrates for sn1 rxns? what nucleophiles are preferred?

A

since carbocations are most stable with more subs, we need a highly substituted substrate. therefore tertiary is most preferred.

nucleophile is normally a weak, non-basic Nu (since the carbocation is a strong electrophile

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7
Q

what is the rate of an sn1 rxn dependent on?

A

only the electrophile since it must lose its LG first before being attacked.

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8
Q

what is a rearrangement? sn1/sn2?

A

rearrangement occurs when a carbocation can become more stable by switching to a different carbon –> only for sn1 though.

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9
Q

aldehydes and ketones: what is PCC used for?

A

to oxidize primary alcohols to aldehydes without over-oxidizing them to carboxylic acids

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10
Q

aldehydes and ketones: what is CrO3, H2CRO4, MnO4 used for?

A

to oxidize secondary alcohols to ketones

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11
Q

aldehydes and ketones: what reagent is used to oxidize tertiary alcohols to carbonyl structures?

A

none! –> tertiary alcohols have no H on their carbon to give up in oxidation

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12
Q

aldehydes and ketones: the hydrogens on the carbons next to the carbonyl carbon are said to be acidic. (CH3COOH). What is an enolate ion / what forms it.

A

In the presence of a strong base (OH or RO-), an alpha proton can be abstracted from a carbonyl compound leaving a carbanion that delocalizes the electron into the conjugated system. This is now an enolate anion which itself can now act as a NUCLEOPHILE due to its carbanion character (check notes for diagram)

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13
Q

aldehydes and ketones: what is an enol?

A

what it sounds like —> an alkene (double bond) next to and alcoh(ol)

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14
Q

aldehydes and ketones: how is an enol formed?

A

First, the enolate ion is formed where a base removes an acidic hydrogen. Then, the O- is protonated to form an alcohol!

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15
Q

aldehydes and ketones: what is keto-enol tautomerism

A

tautomerism is when isomers (in this case, constitutional) exist in equilibrium. thus, when a ketone is in equilibrium with an enol.

tautomerism can cause racemate formation

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16
Q

Why are sodium borohydride / NABH4 and lithium aluminum hydride / LiAlH4 important molecules ?

A

NABH4 and LiAlH4 are strong REDUCING AGENTS (i.e. they get oxidized)

they provide a hydride (a proton + electrons) which is evident since they have hydrogens bonded to elements with low EN (B and Al)

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17
Q

aldehydes and ketones: what occurs when sodium borohydride is mixed with an aldehyde?

A

NaBH4 is a strong reducing agent –> thus, it must reduce the aldehyde to an alcohol

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18
Q

aldehydes and ketones: to make a primary alcohol we need to?

A

reduce an aldehyde (e.g. with NaBH4 or LiAlH4)

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19
Q

aldehydes and ketones: to make a secondary alcohol we need to?

A

reduce a ketone (e.g. with NaBH4 or LiAlH4)

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20
Q

Organometallic reagents: what are these?

A

these are very strong nucleophiles AND bases. they are an R group attached to M+X-

e.g. CH3CH2MgBr

the metal between the halide and the R group causes the R group to become quite negative.

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21
Q

Organometallic reagents: how are they formed?

A

we simply react an alkyl halide (R-X) with a metal such as Mg in an aprotic solvent.

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22
Q

once we have formed an organometallic reagent (CH3CH2MgBr), what can we use it for?

A

organometallic reagents are strong nucleophiles and bases. They are used in nucleophilic addition to add more carbons to a molecule!!

check digram in notes

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23
Q

why cant organometallic reagents be used when the carbonyl group is a carboxylic acid?

A

they will deprotonate the acidic hydrogen instead of acting as a Nu.

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24
Q

how can we prevent an alcohol substituent in a carbonyl compound from undergoing an acid/base rxn with a organometallic reagent? What is used specifically?

A

we first use a protecting group which sequesters the acidic proton. These can be done with tosylates or mesylates

this can be done form amine groups too (NH2)

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25
Q

t or f, mesylate and tosylate are bad LG’s

A

false, they are good leaving groups!

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26
Q

what are acetals and hemiacetals?

A

Acetal - carbon with two OR groups attached and two R groups attached (R can be anything)

Hemiacetal - carbon with one OH and one OR attached. Plus two R groups attached (R can be anything)

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27
Q

Acetals are made from ketones and aldehydes. What rxn occurs to do this? (3 general steps)

A

in general: ketone reacts with alcohol in acid

  1. acid protonates the carbonyl oxygen
  2. alcohol attacks creating the OR group (and the double bond shifts up to the oxygen creating the -OH group)
    - hemiacetal
  3. this is repeated to form an acetal
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28
Q

t or f, the conditions to make an acetal are: a ketone / aldehyde + an alcohol + acid

A

true

29
Q

what is cyanohydrin?

A

when cyanide (C triple bonded to N) attacks a carbonyl centre we form a cyanohydrin. This is similar to a hemiacetal except instead of OR we get CN

30
Q

what are alkyl amines and aryl amines?

A

alkyl amine = an NHn group attached to a sp3 hybridized carbon

aryl amine = an NHn group attached to a sp2 hybridized carbon.

31
Q

what are primary secondary tertiary amines?

A

primary: R-NH2
secondary: R-NH-R
tertiary: R-N-R (-R)

note the lone pair on the N allows amines to act as nucleophiles and bases

32
Q

what is the structure of an imine?

A

essential a ketone / aldehyde but the oxygen is replaced with NR.

R (R1) = N-R

33
Q

how do you form an imine? Reactants and general mechanism? (3)

A

We react an aldehyde or ketone with a PRIMARY AMINE and acid.

  1. acid protonates the oxygen
  2. primary amine attacks
  3. proton transfer allows the alcohol to leave
34
Q

what is the structure of an enamine?

A

double bond connected to an amine.

C=C-NR2

35
Q

how do you form an enamine? Reactants and general mechanism? (3)

A

we react an aldehyde or ketone with a SECONDARY AMINE (unlike imine which is primary)

  1. we form an imine, but the NR2 which is double bonded to the carbonyl carbon cannot be deprotonated.
  2. so a base is used to deprotonate an acidic hydrogen which then forms a double bond and alleviates the amines positive charge.

check notes diagram.

36
Q

true or false, both imine and enamine formation is favoured in mildly acidic conditions.

A

true

37
Q

t or f, enamines are electrophilic

A

false, they are nucleophilic due to their imine ion resonance forms.

C=C double bond jumps onto the alpha carbon forcing the lone pair in the nitrogen to form a double bond with the carbonyl carbon (imine)

38
Q

since enamines are nucleophilic, how to they react with alkyl halides and acid?

A

they attack the R of R-X via the nucleophilic carbon. followed by acid it creates a ketone / aldehyde (the reverse process of making it the enamine)

39
Q

what is an aldol condensation reaction?

A

this is when two ketones / aldehydes react together (often the same molecule reacting with itself) in the presence of a strong base.

40
Q

what is the general mechanism of an aldol condensation?

A

a strong base will deprotonate the acidic alpha hydrogen of one of the ketones. Then the negative carbon will act as a nucleophile to attack the carbonyl centre of the other ketone.

41
Q

what is a crossed aldol condensation?

A

this is when the two reactants are not the same molecule. often one of these molecules will be chosen so that is LACKS ACIDIC PROTONS so that it can only act as an electrophile –> avoid racemate.

42
Q

t or f, if a asymmetrical ketone is used to become the enolate ion (which then reacts with any other ketone), there may be more than one product depending on which side of the ketone has a proton abstracted.

A

true - check notes

43
Q

aldol condensation: what is the thermodynamic product?

A

In a aldol condensation, the first step is the abstraction of an acidic proton creating an alkene bond. More substituted alkenes are more stable and therefore, the more substituted side of the ketone is more thermodynamically favourable

thermodynamic control –> use an unhindered base (NaOH) at room temperature

44
Q

aldol condensation: what is the kinetic product?

A

the kinetic product is the faster forming product in the right conditions.

kinetic control –> use a large bulky base and run it at low temperatures (less T means less energy to battle through steric hindrance) –> -78C

45
Q

what base is chosen for aldol kinetic control?

A

LDA which is bulky.

46
Q

the product of an aldol condensation is a beta-hydroxy-aldehyde (check notes). This actually undergoes intramolecular hydrogen bonding between the beta-OH and the carbonyl O. however, if we heat this molecule up enough, what occurs?

A

Elimination rxn = dehydration reaction

water is lost and we create an alpha,beta-unsaturated carbonyl compound

47
Q

what is the product of an aldol condensation then elimination reaction

A

alpha,beta-unsaturated carbonyl compound

note: the double bond is always between alpha-beta

48
Q

recall that lithium aluminum hydride and sodium boron hydride can reduce ketones to 2 alcohols and aldehydes to 1 alcohols. What is true of these reducing agents for reducing carboxylic acids?

A

NaBH4 CANNOT reduce carboxylic acids

LiAlH4 CAN reduce carboxylic acids

49
Q

what reagent will not work in oxidizing a primary alcohol to a carboxylic acid?

A

PCC -> stops at aldehyde

50
Q

what reaction is prone to occurring when a carbonyl group is beta to a carboxylic acid group?

A

decarboxylation –> i.e. the loss of CO2

51
Q

what is esterification? what are the reactants + conditions?

A

Esterification is the process of reacting a carboxylic acid with an alcohol to form an ester IN THE PRESENCE OF ACID

52
Q

in general what is the mechanism of esterification? (3)

A

1) acid protonates the carbonyl =O
2) the alcohol attacks
3) acid the protonates a hydroxyl group which kicks off the other hydroxyl group

check notes

53
Q

what is the reaction called if we want to make an alcohol and a carboxylic acid from an ester (reverse of esterification)

A

hydrolysis

54
Q

acid catalyzed ester hydrolysis: explain the first 5 steps

A
  1. H3O+ protonates the carbonyl =O
  2. conjugate acid water attacks the carbonyl =OH+ (which breaks the =OH bond to -OH)
  3. then the OR group is protonated by the acid
  4. then the -OH forms a double bond which kicks off the ester which creates the original alcohol
  5. deprotonation regenerating H3O+

check notes

55
Q

what is transesterification? what are the reactants?

A

this is when we react a ester with an alcohol. This allows us to replace the current ester R group with the alcohol ester R group

56
Q

t or f, if something is the solvent (depicted as being written above the arrow of reaction) we can assume we have it in excess?

A

true –> this is a way of favouring one ester over another in transesterification

57
Q

base catalyzed ester hydrolysis: explain the first 3 steps

A
  1. strong base (i.e. Nu) attacks the carbonyl =O (note, the other side which is not an ester will likely have no acidic hydrogens)
  2. the carbonyl reforms and kicks off the alkoxide ester group creating a carboxylic acid
  3. the alkoxide is a strong base and therefore deprotonates the carboxylic acid
  4. acid work up must be done to get the carboxylic acid back
58
Q

t of f, all acid or base hydrolysis rxns for carboxylic acid derivatives are the same

A

true

59
Q

what is saponification?

A

the base-mediated hydrolysis of esters that occurs with triglycerides.

the triglyceride is treated with NaOH (any strong base) to give glycerol (an alcohol) and 3 fatty acids (long chain carboxylic acids)

60
Q

how do we make acid halides with Cl or Br?

A

we react a carboxylic acid with
1. SOCl2 for acid chloride

2.PBr3 for acid bromide

61
Q

how do we make anhydrides?

A

we react two carboxylic acids under heat

or by reacting an acid halide with a carboxylic acid

62
Q

how can we make an ester other than esterification (carboxylic acid and alcohol)

A

esters can be prepared from an acid halide, an anhydride, or another ester AND alcohol (for all 3)

63
Q

how can we make an amide?

A

we react an acid halide, anhydride, or ester with the desired amine

64
Q

t or f, amides can be made from carboxylic acids

A

false!! this is the only derivative that cannot be made from a CA.

65
Q

t or f, we can make a CA from any derivative by adding heat and acid

A

true (acid hydrolysis)

66
Q

list the reactivity of CA and its derivatives

A

CA > AH > AA > ES > AM

note: we can use anything more reactive to make anything less reactive (other than using CA to make AM)

67
Q

CA > AH > AA > ES > AM

evaluate the LG’s

A

Cl- is a very good LG since its the conjugate base to a very strong acid

OOR is a good LG (from anhydride) since it displays resonance

RO- is a bad LG since its quite basic

NRH / NH2 / NR2 –> very bad LG, very strong bases

68
Q

t or f, amines are terrible acids

A

true, they are actually good bases

MCAT chem (11 decks)

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