Ch. 14 Alcohols, Ethers, & Thiols

Note: We are skipping Ch 13 HW-14.10, 14.11, 14.13, 14.14, 14.22, 14.33, 14.35,14.37, 14.43, 14.45, 14.54, 14.71

Alcohols, Ethers, & Thiols

• Alcohols and Ethers are two classes of

oxygen

containing organic compounds • Thiols are a class of

Sulfur

organic compounds containing • Thiols are like alcohols, only they have a

-SH

functional group instead of an -OH

Examples

• Alcohol- Ethanol- gas additive, OH alcohol consumed by humans important solvent • Ether- Diethyl ether- 1st anesthetic O important solvent • Thiol- Ethanethiol- additive to natural gas SH

Alcohols

• Functional group is -OH, the hydroxyl group, bonded to a tetrahedral carbon • Nomenclature Rules – Same as for Alkenes and Alkynes except you only drop the -e, and add -ol!

propane propanol OH

Nomenclature of Alcohols

• 1) Select the longest chain that contains the carbon bonded to the -OH group, and number the chain to give the carbon bonded to the -OH group the lowest number •

The -OH group takes precedence over alkyl groups, double bonds, triple bonds, and halogens!!!

Nomenclature of Alcohols

• 2) Change the suffix by dropping the -e, and adding -ol. Use the number to show location. In cycloalkanes, start numbering from the carbon bonded to the -OH.

• 3) Name and number substituents and list them in alphabetical order.

OH

Examples

OH OH Br OH OH

Classification

• We classify alcohols as 1 o , 2 o , and 3 o , depending on the classification of the carbon they are bonded to.

OH OH OH

Multiple -OH’s present

• Molecules with 2 -OH’s are named as diols • Molecules with 3 -OH’s are named as triols • (Note: you do not drop the -e when using diol, triol, etc) • Compounds with 2 -OH’s are refered to as glycols OH OH HO HO OH

Physical Properties of Alcohols

• The most important physical property is their polarity • Both the C-O bond and the O-H bond are polar covalent bonds • Thus alcohols are polar molecules • They also have the ability to hydrogen bond.

• These factors lead to higher B.P’s, M.P’s. etc

Physical Properties of Alcohols

• Because of increase london forces between larger molecules, the B.P. of all types of compounds, including alcohols, increase as molecular weight increases • Alcohols are much more soluble in H 2 O due to their H-bonding capacity.

• As MW increases, the water solubility of alcohols decreases • This is because the hydrocarbon portion of the molecule dominates.

Reactions of Alcohols

A) Acidity of Alcohols -Alcohols are considerably weaker acids than carboxylic acids, but can lose their hydrogen in an acid-base reaction.

OH + Base O + Base-H

Reactions of Alcohols

B) Acid catalyzed dehydration -Converts alcohols to alkenes by eliminating a molecule of water from adjacent carbons When the equivalent of a molecule of water is removed from a compound, it is called a

Dehydration Reaction

Dehydration of Alcohols

• 1 o alcohols are more difficult to dehydrate. They require high temperatures in concentrate H 2 SO 4 • 2 o alcohols require lower temperatures • 3 o alcohols are the easiest to dehydrate and undergo dehydration only slightly above room temperature

OH

Examples

H 2 SO 4 180 o C H 2 C CH 2 + H 2 O OH H 2 SO 4 140 o C + H 2 O OH H 2 SO 4 50 o o C + H 2 O

Prediction of Product

• When the dehydration of an alcohol can yield more than one different double bond, the most substituted double bond will form.

H 2 SO 4 heat + 80% 20% OH

Recap

• Earlier, we learned the acid catalyzed hydration of alkenes OH H acid H 2 C CH 2 + H 2 O H 2 C CH 2 • Now we are saying alcohols can be dehydrated with acid OH H 2 C H CH 2 acid heat H 2 C CH 2 + H 2 O

Equilibrium Reactions

• The fact is that these reactions are reversible • Alkene hydration and alcohol dehydration are competing reactions and the following equilibrium exist: H 2 C CH 2 + H 2 O hydration dehydration OH H 2 C H CH 2

Controlling Equilibriums

• Equilibriums are governed by

Le Chatelier’s Principle

•

Le Chatelier’s Principle-

when external stress is applied to a system at equilibrium, the system will react to relieve the stress

Controlling Equilibriums

• We can control the hydration/dehydration equilibrium by: – Using large amounts of water (favors alcohol formation) – Using concentrated acids (favors alkene formation – Removal of water as it forms (favors alkene formation)

Reactions of Alcohols

C) Oxidation of 1 o and 2 o alcohols -Remember,

oxidation

in organic chemistry is defined as the increase in C-O bonds and/or the decrease in C-H bonds O O [O] [O] OH H OH

Oxidation of Alcohols

• The reagent used for the oxidation of alcohols is potassium dichromate, K 2 Cr 2 O 7 , dissolved in H 2 SO 4 O O OH K 2 Cr 2 O 7 H 2 SO 4 H K 2 Cr 2 O 7 H 2 SO 4 OH • The reaction proceeds to form the carboxylic acid unless the aldehyde is removed

Oxidation of Alcohols

• The oxidation of 2 o ketone OH alcohols results in a K 2 Cr 2 O 7 O H 2 SO 4 • 3 o alcohols can not be oxidized because the carbon bonded to the -OH is bonded to 3 other carbons.

Ethers

• Structure- functional group is a Oxygen bonded to 2 carbons • Simplest ether is dimethyl ether H 3 C O CH 3

Nomenclature of Ethers

• The common naming system is used for simple ethers: – List the alkyl groups bonded to the oxygen in alphabetical order, followed by the work “ether”.

H 3 C O CH 3 O O O O

Physical Properties

• Ethers are polar compounds • The oxygen has a partial minus charge, the carbons bonded to the oxygen have a partial positive charge • Ether have very weak intermolecular forces which results in low boiling points

Reactions of Ethers

• Like alkanes, they are resistant to most chemical reactions • Therefore, they are ideal to use as solvents

Thiols

• The most outstanding property of low molecular weight thiols is their

stench!!

• They are responsible for the wonderful odors from skunks, rotten eggs, sewage, and paper mills • Structure- functional group is -SH, the sulfhydryl group, bonded to a tetrahedral carbon

Naming Thiols

• Name just like alcohols, except add -thiol instead of -ol and don’t drop the -e.

• Find longest chain containing -SH, number to give -SH lowest number.

SH SH SH

Physical Properties

• Very little difference in electronegativity between Sulfur and Carbon, so the bond is actually nonpolar • They

DO NOT

hydrogen bond • They have low boiling points and very little water solubility

Reactions of Thiols

• Thiols are weak acids and react with bases to lose a proton.

SH + NaOH S Na + H 2 O