Chapter9

Chapter 9

Spring 2009

Alcohol nomenclatureusing IUPAC rules

Step 1 Name the longest carbon chain containing the –OH. Change the –e ending to the suffix –ol.

Step 2 Number the carbon chain to give the –OH group the lower number, and apply all other rules of nomenclature.

Step 1: The longest carbon chain above is 5 carbons long, therefore, the base name of the molecule is pentanol.

Step 2: The molecule should be numbered from right to left, placing the –OH group on C2, and the methyl groups on C3 and C4.

The name is thus, 3,4-dimethyl-2-pentanol.

Naming cyclic alcohols

The –OH group is assigned the C1 position. The second substituent then gets the lowest number.

The name of the compound is 3-isopropyl-cyclopentanol

4-ethyl-3-isopropyl-2-heptanol

2-ethyl-5,5-dimethylcyclohexanol

When an alcohol is a substituent

When an alcohol is a substituent

3-(2-hydroxyethyl)cyclohexanol

Ether nomenclature using IUPAC rules

• Name the simpler alkyl group + O atom as an alkoxy substituent by changing the –yl ending of the alkyll group to –oxy. (examples on next page)

• Name the remaining alkyl group as an alkane, with the alkoxy group as a substituent bonded to this chain

Common alkoxy groups

1. Name the longer chain as the alkane and the shorter chain as the alkoxy group

Above the longest chain is a hexane.

Longest chain

2. Use IUPAC rules to finish the name. The substituent should have the smallest number possible. Thus, number from right to left.

3-methoxyhexane

methoxy

2-methyl-1-ethoxycyclopentane

1-butoxy-2,2-dimethylhexane

Cyclic ethers

Naming epoxidesThis is oxirane – the simplest epoxide

These are named as substituted oxiranes1,2-dimethyloxirane1,1-dimethyloxirane

To name an epoxide as a substituent, it is called an epoxy. This is then 1,2-epoxycyclohexane

Preparation of alcohols and ethers

The nucleophile

OH- is commonly supplied as NaOH or KOH

The alkoxide is most commonly generated from its corresponding alcohol.

Draw the product of the following two-step sequence

[1] In the first step the base NaH removes the proton from the alcohol

[2] In the second step of the process the alkoxide acts as a nucleophile displacing the leaving group in an SN2 reaction

Preparation of epoxides

Starting reagent is a halohydrin

Two-step process:

Reactions of…

Alcohols

Dehydration – elimination of –OH and –H from an α and β position to yield an alkene.

Dehydration in Acid

Alcohols undergo dehydration in the presence of a strong acid.

Acids commonly used for dehydration:

p-toluenesulfonic acid TsOH

sulfuric acid

Zaitsev Rule

The more substituted the alkene is the major product when a mixture of constitutional isomers is possible.

Recall the Mechanism for an E1 Reaction

Again, this the kinetically and thermodynamically favored route

Provide the mechanism for the less favored route.

Provide the mechanism for the less favored route.

Also remember, 1° alcohols will react via an E2 mechanism

Unexpected products in an elimination reaction

Reason, more stable carbocations are formed from less stable ones by a shift of a hydrogen atom or alkyl group. The shifts are called 1,2-shifts because:

If R = -CH3 then called a methyl shiftIf H then a hydride shift

Think of the possibilities

A 1,2-methyl shift occurs during the dehydration of this compound

A 1,2-hydride shift occurs during the dehydration of this compound

Used curved arrows to show the methyl shift when 3,3-dimethyl-2-butanol is treated with sulfuric acid.

Used curved arrows to show the hydride shift when 3-methyl-2-butanol is treated with sulfuric acid.

Sometimes it is necessary to use a little more finesse in order to do a dehydration reaction with an alcohol.

Reaction conditions

And what I mean by that is…

You can execute a dehydration reaction with POCl3 and pyradine in order to avoid the use of strong acid.

Conversion of alcohols into Alkyl Halides

R-OH + H-X R-X + H2O

Remember

R-OH + X- R-X + HO- too poor a leaving

group

1° alcohols will react via an SN2 mechanism

2° and 3 ° alcohols will react via an SN1 mechanism

Cl- is the poorest of the halide nucleophiles, and thus, needs help

Let’s not forget stereochemistry

Predict the product(s)

Conversion of alcohols to alkyl chloridesExample:

CH3-CH2-OH + SOCl2 CH3-CH2-Cl + SO2 + Cl-pyradine

Conversion of alcohols to alkyl bromidesExample:

CH3-CH2-OH + PBr3 CH3-CH2-Br + HOPBr2

reaction reagent Usefullness

ROH RCl HCl Can be used for all

ROH RCl SOCl2+ pyridine

Best for CH3OH and 1° and 2° alcohols

ROH RCBr HBr Can be used for all

ROH RCBr PBr3

+ pyridine

Best for CH3OH and 1° and 2° alcohols

ROH RI HI Can be used for all

Conversion of alcohols to tosylates

Reaction:

Mechanism:

Reaction of Ethers with Strong Acids

Example:

Mechanism

Use curved arrows to indicate the mechanism.

Reactions of epoxides

Ring opening with strong nucleophiles

Which way do we go?

In an unsymmetrical epoxide, the nucleophile attacks at the less substituted carbon atom.

Reactions with acids – HX

The nucleophile adds here to the more substituted carbon because it is more able to accept a partial positive charge.