17- Alcohols

Alcohols

have an OH connected to a saturated C, sp3

Phenols

has an OH group connected to C in benzene ring

Enols

OH groups bonded to vinylic, sp2 hybridized Cs

IUPAC rules for alcohols

• use ol suffix

• select longest C chain containing OH group

• number chain from end nearer the OH group

• list susbtituents alphabetically

Cyclic Alcohols

use cyclo, OH assumed on C1

Phenol Naming

use phenol as parent name, name substituents by position on OH

Structural Characteristics

• similar structure to water, sp3

• higher bp & bp than hydrocarbons

• can H bond

Acid-Base Behavior of Alcohols

Alcohols can act as weak Bronsted acids or bases; they can donate or accept protons

What is the acidity of alcohols affected by?

affected by number of alkyl substituents, presence of EWGs

Acidity Comparison of Alcohols

phenols (pKa 10), more acidic than alcohols (pKa 16), phenols form salts with NaOH

Preparation Techniques for Alcohol Synthesis

• hydration of alkenes

• reduction of carbonyl compounds

• grignard reagent reactions

Hydration of Alkenes

• hydroboration/oxidation

• oxymercuration/reduction

Reduction of Carbonyl Compounds

• Reagents: NaBH4, LiAlH4

• reduces aldehydes ketones, carboxylic acids, esters

Grignard Reagent Reactions

• reacts with carbonyl compounds

• produces different alcohol types based on carbonyl structure

Key Alcohol Reaction Types

• conversion to alkyl halides

• tosylate formation

• dehydration to alkenes

• ester formation

• oxidation

Oxidation Rules

• primary → oxidized to aldehydes or carboxylic acids

• secondary → oxidized to ketones

• tertiary → CAN’T be oxidized

Alcohol Protection

• protect interfering functional group

• perform desired reaction

• deprotect group

What is a common protecting group?

TMS ether

Phenol Specific Reactions

• ester formation

• ether formation

• electrophilic aromation substitution

• oxidation to quinones

Alcohol IR

OH → 3300-3600

CO → 1050

Alcohol NMR

13C NMR → C-OH at 50-80

1H NMR → OH at 3-8

Alcohol Mass Spectroscopy

alpha cleavage, dehydration to alkene radical anion

What is the relationship between the number of alkyl substituents on an alcohol and its acidity?

more alkyl substituents decrease acidity

How many times can a primary alcohol be oxidized, & what is the final product?

2x, with the final being a carboxylic acid

What mechanism do tertiary alcohols follow when converted to alkyl halides with HCl or HBr?

SN1 mechanism through carbocation intermediate

Why do phenols have much lower pKa values (around 10) compared to alcohols (around 16)?

phenoxide ion is stabilized by resonance with the benzene ring, which allows for greater delocalization of the negative charge.

Why is triethylamine used as the base when forming TMS ethers to alcohol protection?

prevents formation of alkoxide ion while still removing HCl

Why can’t Grignard reagents be prepared from molecules containing alcohol functional groups?

alcohol OH group reacts w/ Grignard reagent in an acid-base reaction

What is the primary structural difference between alcohols & phenols?

alcohols have OH groups connected to saturated C while phenols have OH groups connected to benzene ring C

What is the major advantage of using POCl3 in pyridine to alcohol dehydration compared to acid-catalyzed dehydration?

works at lower temperatures & is gentler on sensitive molecules

What is the primary reason why alcohols & phenols have much higher mp & bp than similar hydrocarbons

they can form H bonds in the liquid state

What is the IR absorption range for OH stretching in alcohols & phenols?

3300-3600

What happens when a Grignard reagent reacts with a carboxylic acid instead of a carbonyl compound?

it undergoes an acid-base reaction producing a hydrocarbon

What reagent would you use to oxidize a primary alcohol to an aldehyde while preventing further oxidation to a carboxylic acid

PCC (pyridinium chlorochromate) in CH2Cl2

Why do phenols react with NaOH solutions while alcohols do not?

phenols are more acidic than alcohols, allowing them to react with NaOH to form phenoxide ions.

What type of mechanism does the dehydration of alcohols with POCl3 follow?

E2 mechanism via an intermediate ester

What type of alcohol is produced when a Grignard reagent reacts with formaldehyde?

A primary alcohol

Which reagent combination would be most appropriate for converting a ketone to a secondary alcohol without affecting other reducible groups in the molecule?

NaBH4 in ethanol

Which type of alcohols require the harshest conditions for acid-catalyzed dehydration?

primary alcohols

How many equivalents of Grignard reagent are required to react with an ester to form a tertiary alcohol?

2 equivalents

What is the correct IUPAC naming priority order for alcohols?

OH group has higher priority than both C=C & C=-C bonds

In the formation of tosylates from alcohols what happens to the stereochemistry at a chiral center?

it is retained

How do alcohols act as acids?

transfer a proton to water, make H3O & alkoxide/RO or phenoxide/ArO

Acidity Constant

Ka, measures extent to which Bronsted acid transfers proton to water

pKA

pKa= -log Ka

more pKa = less strength

How do substituents affect acidity of alcohol?

• more alkyl substituents = lower acidity

• more alkyl substituents inhibits solvation of alkoxide = equilibrium favors reactants

• more EWG = more acidity, stabilize alkoxide ion, delocalize electron charge

What bases are used as reagents?

alkoxides

How can alcohols form an alkoxide?

weak acid, requires a strong base to form an alkoxide such as alkali metals to form metal alkoxides, named by adding -ate

Alcohol MS

alcohosl do an alpha cleavage, C-C bond nearest hydroxyk is broken, yielding a neutral radical & charged O fragment

How do alcohols yield an alkene radical reactions?

undergo dehydration

Alcohol & Phenol Spectroscopy

• IR = OH stretch at 3600 except if H bonded then 3300-3400, C-O at 1050, phenol OH at 3500

• 13C NMR = C-OH at 50-80

• 1H NMR = EWG effect of O, 3.5-4, phenol O-H at 3-8