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Chemistry 6-10 > Module 8 The Chemistry of Alcohols > Flashcards

Module 8 The Chemistry of Alcohols Flashcards

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1
Q

Alcohols posses a ______ ______.

A

Hydroxyl Group (-OH)

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2
Q

Like Halides Alcohols are classified by the type of _____ they are attached to.

A

Carbon.

Primary, Secondary , Tertiary.

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3
Q

Alcohol attached to a benzene ring.

A

Phenol.

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4
Q

Physical Properties of Alcohols.

A

Higher Boiling Point due to to H-Bonding.

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5
Q

Acidity of Alcohols and Phenols.

A

A strong base is necessary to deprotenate an alcohol.

NaH is often used to generate the corresponding alkoxide.

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6
Q

Alkoxide.

A

Conjugate Base of an Alcohol.

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7
Q

Which is more acidic Phenol or Cyclohexanol?

A

Phenol because its conjugate base is stabilized by resonance.

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8
Q

Induction and acidity.

A

Presence of electron withdrawing groups increases the acidity of an alcohol or phenol.

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9
Q

Solvation and Acidity.

A

The more poorly solvated a conjugate base, the less stable it is and the less acidic it’s conjugate acid.

more sterically hindered the molecule the less able the solvent is to stabilize th charge.

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10
Q

Alcohols can be synthesized from alkyl halides via ______ and alkenes via ______, using what reagents?

A

Substitution:
SN1: H2O (secondary;tertiary)
SN2: NaOH (primary)

Addition:
H2SO4 (dilute/ with water)
BH3

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11
Q

A third method for generating alcohols.

A

The reduction of Carbonyl compounds (C=O)

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12
Q

Converting a carbonyl to an alcohol requires a ________ agent.

A

Reducing Agent.

It is oxidized in the process while the carbonyl is reduced.

The reduction of the carbonyl group, overall, results in the addition of H-H across the pi bond.

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13
Q

Carbonyl Properties.

A

Has Resonance

The carbonyl carbon is very electrophilic, it doesn’t have electron density.

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14
Q

Quick Note on determining Oxidation States.

A

To determine the oxidation state of an atom, imagine the electrons in a bond as a lone pair on the more electronegative atom

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15
Q

Oxidation RXN vs Reduction RXN.

A

Ox rnx: Carbon’s oxidation state is increased (0 to +2). Loss of electrons.

Red rxn: Carbon’s oxidation state decreases (+2 to -1) Gaining electrons.

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16
Q

Catalytic Hydrogenation.

A

H2, Pt, Pd or Ni.

Adding Hydrogen across the double bond.

This method is rarely used; high temperature and pressure is required.

Also not very selective hydrogen will add across any double bond present.

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17
Q

Sodium Borohydride (NaBH4)

A

Common Reducing Agent for aldehydes and ketones.

Hydrogen in borohydride reacts as if it is an H-.

The H- is a good nucleophile that can attack the carbon, leaving oxygen with a formal negative charge.

This oxygen is then protenated giving the final product.

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18
Q

Lithium Aluminum Hydride.

A

LAH

Hydrogen behaves like H- and attacks the carbonyl carbon.

Electrons will go to the oxygen, then a proton donor H2O is added in the second step to reduce the negatively charged oxygen.

The Al-H bond is a much stronger H- donor because of that we can’t have any acidic hydrogen in the reaction when we add the aluminium hydride, it will react as an acid base reaction.

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19
Q

Which is more reactive LAH or NaBH4

A

Aluminium is much less electronegative than boron, the bond between Al-H is much more polarized. LAH is much stronger H- donor than BH4.

BH4 is a much weaker H- donor so we can use H2o or alcohol as a solvent.

LAH can’t have H2O or OH as a solvent.

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20
Q

When ______ ketone is reduced a new _____ ____ is created and a pair of ______ is obtained.

A

Asymetrical.

Chiral Center

Stereoisomers.

SP2 hybridized flat carbonyl compound, H- nucleophile can attack from either face of the carbonyl group.

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21
Q

Key difference between Hydride delivery (LAH;NaBH4) and hydrogenation.

A

Hydride delivery agents are selective for the carbonyl group whereas hydrogenation is not.

Catalytic hydrogenation reduces the carbonyl and the alkene.

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22
Q

Note about hydride delivery reactivity.

A

Can be fine tuned by using derivatives with varying R-groups.

changing the R group makes the hydride either more electron withdrawing or donating, more or less reactive.

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23
Q

_____ is strong enough to reduce ester and carboxylic acids whereas ____ is generally not.

A

LAH.
NaBH4.

Reduction of carboxylic acids and esters yeild primary alcohols.

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24
Q

Reduction of Esters.

A
  • Requires 2 equivalents of hydride.
  • the first two steps result in the formation of an aldehyde, which is then reduced to an alcohol.
  • Methoxide is nor a good leaving group, but acts like one here because the high energy intermediate becomes a stable aldehyde.
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25
Q

How are Grignard Reagents formed?

A
  • To form a Grignard Reagent an alkyl halide is treated with Mg.
  • Mg inserts itself between the halide and the carbon.
  • Carbon now behaves like C- because carbon is more electronegative than magnesium.
  • A carbon with a formal negative charge is a good base and nucleophile.
  • Reactions are performed in ether because we can’t use C- in the presence of a proton source,
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26
Q

Grignard Reagent Mechanism.

A
  • Grignard Reagent acts as a nucleophile and will attack the carbonyl group of a ketone or an aldehyde.
  • Grignard Reaction will give a new C-C
  • We add water in the second step because C- very basic.
27
Q

Grignard Reagents and esters.

A

Grignard reagents in excess will react with esters to create a tertiary alcohol.

28
Q

Grignard Reagents generate….?

A

New C-C bonds! Always determine which C-C bonds in the product can be new.

29
Q

Why do we use anhydrous ethers as solvents for Grignard reactions?

A

Because Grignard reagents are both a strong bases and strong nucleophiles, care must be taken to protect it from exposure to water or alcohols.

30
Q

Protection of Alcohols

A

When we want to use an alkane bonded to an -OH group as a grignard reagent the -OH needs to be protected or the C- minus generated will self react with the alcohol to reduce itself back to CH.

31
Q

Three-Step Process is required to achieve the desired synthesis. (Alcohol Protection)

A

(1) Protect alcohol
(2) Perform Grignard Reaction
(3) Deprotect

Protect the alcohol by converting it to a functional group that does not contain a hydrogen.

One such protecting group is trimethylsilyl (TMS)

The TMS protection step requires the presence of a base to neutralize the 0HCl byproduct (EtN3)

32
Q

Halogenation Reactions of Alcohols.

A
  • Tertiary alcohols are converted to alkyl halides with H-X (SN1 rxn)
  • Primary and Secondary Alcohols occur via SN2
33
Q

To make an alkyl chloride (_ _ _ _) must be used with HCl.

A

ZnCl2.

The zinc cation is required to make -OH a better leaving group.

Chloride is not as good as a nucleophile than bromine.

34
Q

-OH can be converted to a good leaving group by a _______ _____.

A

Tosyl Group.

Tosylate can then undergo SN2 substitution to obtain an alkyl halide.

35
Q

Primary and Secondary alcohols can also be converted to alkyl Halides using _ _ _ _ or _ _ _

A

SOCl2
PBr3

other methods have complications and competing products.

35
Q

Primary and Secondary alcohols can also be converted to alkyl Halides using _ _ _ _ or _ _ _

A

SOCl2
PBr3

other methods have complications and competing products.

36
Q

Chlorination of Alcohols with SOCl2

A

Sulfur is very electrophilic with a formal positive charge. Oxygen attacks the sulfur, the intermediate sulfur can reform the double with oxygen by ejecting chloride. This forms a good leaving group, and generates the chloride nucleophile.
This reaction is not reversible, because it forms SO2 gas that dissipates

37
Q

Bromination of Alcohols with PBr3

A

Phosphorus tribromide is a lewis acid, the oxygen attacks the phosphorus, bromide leaves the molecule, bromide then acts as a nucleophile to attack the carbon bonded to the oxygen, the oxygen is now a good leaving group and will leave to form the final alkyl halide,

No competing products and the reaction is not reversible so we go to the products.

38
Q

Elimination Reactions of Alcohols

A

If the alcohol is converted into a better leaving group, then a strong base can be used to promote E2.

E2 reactions generally preferred over E1 reactions. E2 pathway not susceptible to rearrangements, and regioselectivity can be controlled (small, strong versus big bulky strong base)

39
Q

Alcohols undergo elminations in ______ conditions.

A

Acidic.

protenate the alcohol, making it aa good leaving group. We can then form the carbocation in acid we have no nucleophile around other than water just made by the leaving group. Water acts as a base and deprotonates the carbon to fomr an alkene under an E1 reaction,

40
Q

Oxidation of Alcohols

A

We oxidate alcohols to from carbonyl groups.

41
Q

Oxidation of primary alcohols proceed to an ________, and then to the ______ _____.

Oxidation of secondary alcohols produces a ______.

A

An aldehyde.
carboxylic acid.

ketone.

Eliminating H2 from the molecule.

[O] = a general oxidant.

Ketones can’t be further oxidized.

42
Q

Why can’t tertiary alcohols undergo oxidation reactions?

A

Because they do not have any proton at the alpha-position.

43
Q

Most common oxidizing agent used to oxidize alcohols _ _ _ _ _.

A

Chromic Acid in aqueous acid.

Formed from NaCrO or CrO3

44
Q

Oxidation of the alcohol with chromic acid involves…

A

(1) Formation of a chromate ester.
(2) Elimination to form the pi bond.

  • OH attack Cr.
  • Water leaves
  • Chromate ester is formed
  • Water acts as a base and deprotenates the carbon bonded to oxygen.
  • Electrons from the hydrogen form
45
Q

Chromic acid will oxidize a primary alcohol to a ______.

PCC can be used as the oxidant to produce an ________.

A
  • Carboxylic Acid.
  • Aldehyde.

To stop the reaction at the aldehyde we use a less strong oxidizing reagent.

46
Q

Chromic Acid or PCC will always convert a secondary alcohol to a _______.

A

Ketone.

47
Q

Alternatives to chromium as an oxidizing agent.

A

Dimethyl sulfoxide (DMSO) and oxalyl chloride form the active oxidant.

Swern Oxidation.

  1. Chlorodimethyl sulfonium ion (Active Oxidant) is attacked by the alcohol and ET3N facilitates elimination to form the pi bond.
    - ET3N removes three protons.
    - a methyl group attached to sulfur loses a proton generating a carbanion that will attack the proton bound to the alpha-carbon
    - the electrons from that proton form the double bond between the a;pha carbon and oxygen
    - Dimethyl sulfide is ejected.
  • 2˚ alcohols are oxidized to ketones
  • the Swern oxidation converts 1˚ alcohols to aldehydes
48
Q

Ether Group

A

Includes an Oxygen atom bonded to two alkyl groups.

-R groups can be alkyl, aryl or vinyl groups (not acyl groups)

Symmetric = same alkyl groups,
Asymmetric = different alkyl groups
49
Q

Structure and Properties of Ethers.

A
  • Alcohols have relatively high boiling points as a result of H bonding.
  • ethers can act only as H-Bond acceptors.
50
Q

Why are Ethers common solvents for organic reactions?

A
  1. Low BP (Easy to evaporate)
  2. fairly unreactive
    - Because ethers have no acidic hydrogens as part of the functional group and the oxygen’s lone pair is surrounded by -R groups making them less nucleophilic due to sterics.
51
Q

Williamson Ether Synthesis.

A

-Viable approach for asymmetrical ethers.

  • 1)NaH
  • 2)RX

A hydride ion functions as a base and deprotenates the alcohol.

The resulting alkoxide ion then functions as a nucleophile and attacks the alkyl halide in an SN2 process.

SN2: only works well with unhindered (primary and methyl) alkyl halides.

52
Q

Acid-Catalyzed addition to alkenes to synthesize ethers

A

Can be used to synthesize ethers.

Similar to the reaction of adding water across the double bond.

ROH

53
Q

Acid-Promoted Cleavage of Ethers.

A
  • H-X
  • R-O-R

Oxygen acts as a base and deprotenates the acid.

The R group is then attacked by the resulting halide nucleophile.

R group leaves in an SN2 reaction. The electrons return to oxygen generating an alcohol.

This method can be repeated again to eject the second R group and form water.

54
Q

Epoxide

A

Reactive form of Ether.

Epoxides can be opened by many other strong nucleophiles as well.

Ring opening of an epoxide is regioselective and stereoselective

54
Q

Epoxide

A

Reactive form of Ether.

Epoxides can be opened by many other strong nucleophiles as well.

Ring opening of an epoxide is regioselective and stereoselective

55
Q

Ring-opening Reactions of Epoxides

A

Regioselectivity - Epoxide ring-opening is SN2, and so the least hindered carbon is more reactive towards a strong nucleophile.

Stereoselectivity – as with SN2, inversion of configuration is observed.

56
Q

Ring-opening Reactions of Epoxides (primary;secondary;tertiary carbon)

A

Under acidic, conditions, Nu attack still occurs at the less substituted carbon, if a 1˚ and a 2˚ carbon are present

But if a 3˚ carbon is present, then the Nu attacks there:

  • Since the epoxide is cationic under acidic conditions, electronic effects result in the C-O bond to a 3˚ carbon to be much weaker
  • But, if a 3˚ carbon is not present, then steric effects dominate
57
Q

Thiols and Sulfides

A

Thiol (-SH group): sulfur analog of an alcohol

SH H is more acidic, SH is a weaker base, promotes SN2 instead of E2

The hydrosulfide ion (HS-) is a strong nucleophile and a weak base

HS- promotes SN2 rather than E2

58
Q

____ and ____ thiols can be synthesized from ___ reaction of NaSH with an alkyl halide.

A

primary
secondary
SN2

Stereospecific, leads to inversion if it’s a chiral center.

We don’t need to worry about competing SN2 reactions.

59
Q

Thiols are oxidized to ______ with _ _ under basic conditions

A

Disulfides
Br2

We deprotenate the first thiol because it’s a decent acid, the s- thiol reacts in an SN2 reaction with the Br2, the second S- attacks the the sulfur bonded to the bromine, bromine leaves, generating the disulfide.

Another way to generate a disulfide is to leave the thiol out in the open or to add hydrogen perioxide since they are easily deprotenated

60
Q

Sulfur analogs of ethers are called ____ or ____

A

Sulfides
Thioesters.

Sulfides are generally prepared by nucleophilic attack of a thiolate on an alkyl halide

61
Q

Sulfides can also be _______ to sulfoxides or sulfones.

A

The choice of oxidizing agent depends on whether one needs to make a sulfoxide or a sulfone

Chemistry 6-10 (4 decks)

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