SNS - Organic Chemistry - Alcohols and Ethers

Question 1

Alcohols

Physical Properties

1. Boiling Points
2. Solubility

Answer 1

1. Significantly higher than those of analogous hydrocarbons due to hydrogen bonding. Molecules with more than one hydroxy group show greater hydrogen bonding
2. Increased due to hydrogen bonding. Phenol is slightly soluble in water, presumably due to hydrogen bonding

Question 2

Alcohols

Physical Properties

Acidity

Answer 2

The -OH hydrogen is weakly acidic and alcohols can dissociate into protons and alkoxy ions.

-OH hydrogens of phenols are more acidic due to resonance stabilisation of the anion. Consequently, these compounds form intermediate hydrogen bonds and have relatively high melting and boiling points.

Acidity increases as more alkyl groups are attached as the electron donating alkyl groups destabilise the alkoxide anion. Electron withdrawing groups stabilise the anion and increase acidity. Phenol more acidic than aliphatic alcohols

For phenols, the presence of other substituents on the ring has significant effects on these properties. Electron withdrawing susbstituents increase acidity and vice versa

Question 3

Alcohols and Ethers

Key Mechanisms of Formation

Answer 3

1. Nucleophilic Substitution: (a) SN1, (b) SN2
2. Electrophilic addition to a double bond
3. Nucleophilic addition to a carbonyl

Question 4

Alcohols

Synthesis

Answer 4

1. Addition
2. Substitution
3. Reduction
4. Phenol

Question 5

Alcohols

Synthesis

Addition

Answer 5

Can be prepared via several reactions involving the addition of water to a double bond

Can also be prepared via the addition of organometallic compounds to carbonyl groups

Question 6

Alcohols

Synthesis

Substitution

Answer 6

SN1 and SN2 can be used to prepre alcohols from alkyl halides under the proper conditions

Question 7

Alcohols

Synthesis

Reduction

Answer 7

Of aldehydes, ketones, carboxylic acids and esters

LlAlH4 or NaBH4 used as a catalyst

Question 8

Alcohols

Synthesis

Phenol

Answer 8

Can be synthesised from arylsulphonic acids with hot NaOH. However, useful only for phenol and its alkylated derivatives as most functional groups are destroyed by the harsh conditions

Can also be synthesised via hydrolysis of diazonium salts

Question 9

Alcohols

Reactions

Answer 9

1. Elimination
2. Oxidation
3. Substitution

Question 10

Alcohols

Reactions

Elimination

Answer 10

Can be dehydrated in a strongly acidic solution (usually H2SO4) to produce alkenes

E1 mechanism, proceeds via protonated alcohol

Yields two main products, the major being the more stable of the two alkenes (nonterminal). This occurs via movement of a proton to produce the more stable secondary carbocation

Question 11

Alcohols

Reactions

Substitution

SN1

Answer 11

Displacement of OH- by substitution reactions is rare as it is a poor leaving group

For these reactions, the -OH group must therefore first be transformed into a good leaving group.

Protonation of the alcohol make water the leaving group, which is good for SN1 reactions

Question 12

Alcohols

Reactions

SN2

Answer 12

Conversion of the -OH into tosylate group is excellent for SN2 reactions

A common method of converting alcohols into alkyl halides involves formation of inorganic esters which readily undergo SN2 reactions. Alcohols react with thionyl chloride to produce an intermediate inorganic ester (a chlorosulphide) and HCl. The Cl- of HCl displaced SO2 and regenerates cl- to form the desired alkyl chloride. An analogous reaction in which the alcohol is treated with PBr3 instead of thionyl chloride produces alkyl bromides

Question 13

Alcohols

Reactions

Electrophilic Substitution

Answer 13

Phenols readily undergo electrophilic aromatic substitution reactions as have lone pairs that it can donate to the ring

The -OH is strongly activating, ortho/para directing substituent

Question 14

Alcohols

Reactions

Oxidation

Answer 14

Generally involves the formation of some form of chromium (VI) as the oxidising agent which is reduced to chromium (III) during the reaction.

Tertiary alcohols can’t be oxidised for valence reasons

Treatment of phenols with oxidising agents produces quinones

1. PCC (pyridinium chlorochromate) - commonly used as a mild oxidant. Converts primary alcohols to aldehydes without overoxidation to the acid. In contrast KMnO4 can a very strong oxidising agent takes the alcohol all the way to the carboxylic acid. PCC can also be used to form ketones from secondary alcohols
2. Alkali potassium/sodium dichromate salts - Oxidises secondary alcohols tp ketones and primary alcohols to carboxylic acids
3. CrO3 - stronger oxidant. Often dissolved in Jones reagent (dilute H2SO4 in acetone). Oxidised primary alcohols to carboxylic acids and secondary alcohols to ketones

Question 15

Ethers

Answer 15

Compound with two alkyl (or aryl) groups bonded to an oxygen atom

General formula ROR

Question 16

Ethers

Nomenclature

Answer 16

Named as alkoxyalkanes

Smaller chain used as prefix and larger as suffix

Exceptions are cyclic ethers, some of which have common names such as epoxide

Question 17

Ethers

Physical Properties

Answer 17

Don’t undergo hydrogen bonding as have no hydrogen atoms bonded to oxygen atomes

Therefore boil at relatively low temp compared to alcohols and at approxiametely the same temp as alkanes of comparable MW

Only slighly polar and therefore only slightly soluble in water

Relatively inert to most organic reagents and are frequently used as solvents

Question 18

Ethers

Synthesis

Williamson

Answer 18

Williamson ether synthesis produces ethers from the reaction of metal alkoxides with primary alkyl halides

The alkoxides behave as nucleophiles and displace the halide or tosylate via SN2 reaction to produce an ether

Alkoxides will attack only nonhindered halides. Thus to synthesize a methyl ether, an alkoxide must attack a methyl halide. The reaction can’t be accomplished with methoxide ion attacking a hindered alkyl halide substrate

Can also be applied to phenols. Relatively mild conditions are sufficient die to phenol’s acidity

Question 19

Ethers

Synthesis

Cyclic Ethers

Answer 19

Prepared in a number of ways

Oxiranes can be synthesised by means of an internal SN2 displacement

Oxidation of an alkenewith a peroxy acid (RCOOOH) such as MCPBA will also produce an oxirane

Question 20

Ethers

Reactions

Answer 20

1. Cleavage
2. Peroxide Formation

Question 21

Ethers

Reactions

Peroxide Formation

Answer 21

Ethers react with O2 in air to form highly explosive compounds - peroxides (ROOR)

Question 22

Ethers

Reactions

Cleavage

Answer 22

Cleavage of straight chain ethers will occur only under vigorous conditions: usually at high temp and in the presence of HBr or HI

1. Initiated by the protonation of the ether oxygen
2. Proceeds via SN1 or SN2 depending on conditions and the structure of the ether
3. The alcohol products usually react with a second molecule hydrogen halide to produce an alkyl halide

Question 23

Ethers

Reactions

Cleavage

Epoxides

Answer 23

Since epoxides are highly strained cyclic ethers, are susceptible to SN2.

Unlike straight chain ethers which are catalysed by an acid only, these reactions can be catalysed by an acid or base.

In symmetrical epoxides, either carbon can be nucleophillically attacked, but in asymmetrical epoxides, the most substituted carbon is nucleophilcally attacked in the presence of acid and the least substituted carbon in the presence of a base.

Base catalysed cleavage has the most SN2 character so occurs at the least hindered (least substituted) carbon. The basic envoronment provides the best nucleophile

Acid catalysed cleavage thought to have some SN1 character as well as some SN2 character. The epoxide -O can be protonated to make it a better leaving group. This gives the carbons a slight positive charge. Since substitution stabilises the positive charge the more substituted C becomes a good target for nucleophilic attack