Alcohols

Question 1

What is an alcohol?

Answer 1

Any organic compound in which a hydryoxyl group (-OH) is bound to a carbon atom of an alkyl or substituted alkyl group

Question 2

What is the system for naming alcohols?

Answer 2

By replacing the -e of the corresponding alkane with -ol.

Special names are used for branched groups such as 2-propanol

Question 3

What is a short hand for methanol? Ethanol?

Answer 3

MeOH

EtOH

Question 4

For acohols, Ka = 10^-18, what does this mean?

Answer 4

They are very weak acids, weaker than water.

The conjugate bases are called alkoxides, very little of which can be found in solution

Question 5

What impact does the number of attached carbons have on the acidity of an alcohol?

Answer 5

Acidity decreases with increasing number of attached carbons

Question 6

How does the boiling point and solubility of alcohols compare to alkanes, alkenes, aldehydes, ketones, alkyl halides?

Answer 6

Greater boiling point and solubility.

This is due to the greater polarity and hydrogen bonding of the alcohol. Hydrogen bonding being a weak association of the -OH proton of 1 molecule with the oxygen of another. Both donor and acceptor are required. Sometimes an atom can act as both, such as in a chain

Question 7

What is “dehydration” of alcohols and what does it produce?

Answer 7

Loss of water molecule from alcohol molecule. Produces alkene.

In order to create some of these reactions, the temperature must be between 300 and 400 degrees celsius, and the vapours must be passed over a metal oxide catalyst. Alternatively, strong, hot acids, such as H2SO4 of H3PO4 at 100 to 200 degrees celsius may be used

Question 8

In organic chemistry, oxidation is what? What is reduction?

Answer 8

The increasing of oxygen or decreasing of hydrogen content. Reduction is the opposite

Question 9

What occurs in a substitution reaction?

What are the two different types of substitution reactions?

Answer 9

One atom or group is substituted for another atom or group. Sn1 and Sn2.

For an alcohol, the -OH group is substituted by a halide (usually chlorine or bromine)

Question 10

What happens in the Sn1 (1st order or monomolecular nucleophilic substitution) reaction?

Answer 10

The transition state involves a carbocation, the formation of which is the rate determining step.

Alcohol substitutions that proceed by this mechanism are those involving benzyl groups, allyl groups, tertiary and secondary alcohols.

• The reaction is first order; the rate depends on RL where R represents an alkyl group, and L represents a substituent or ligand.
• There is a racemization of configuration, when a chiral molecule is involved.
• A stable carbonium ion should be formed; thus in terms of reaction rat. benzyl groups = allyl groups > tertiary alcohols > secondary alcohols&raquo_space; primary alcohols
• The stability of alkyl groups is as follows: primary alkyl groups < secondary alkyl groups < tertiary alkyl groups

Question 11

What is the mechanism of the Sn2 (2nd order or bimolecular nucleophilic substitution) reaction?

Answer 11

• The reaction rate is second order overall (rate depends on concentration of two compounds).
• The nucleophile adds to the alkyl group by backwards replacement. Thus optically active alcohols react to give an inversion of configuration forming the opposite enantiomer.
• Large or bulky groups near or at the reacting site may hinder or retard a reaction. This is called steric hinderance.
• The substitution reactions for methanol and other primary alcohols are by the Sn2 reaction mechanism

Question 12

Why are steric factors important?

Answer 12

Size or steric factors are important since they affect Sn2 reaction rates; in terms of reaction rate, CH3- > primary alcohols > secondary alcohols&raquo_space; tertiary alcohols

Question 13

When do elimination reactions occur?

Answer 13

When an atom or a group of atoms is removed (eliminated) from adjacent carbons leaving a multiple bond.

There are two different types of elimination reactions, E1 and E2. In the E1 (elimination 1st order) reaction, the rate of reaction depends upon the concentration of one compound. E1 often occurs as minor products alongside Sn2 reactions. E1 can occur as major products in alkyl halides or to an alcohol.

E2 (elimination second order) reactions, the rate of reaction depends on the concentration of two compounds. E2 reactions require strong bases like KOH or the salt of an alcohol (i.e. sodium alkoxide). An alkoxide can be synthesised from an alcohol using either Na or NaH as reducing agents

Question 14

How is the quality of a leaving group determined?

Answer 14

Good leaving groups have strong conjugate acids