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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.