Org 6 - Alcohols

Question 1

What is an alcohol ?

Answer 1

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

Question 2

What is the general formula of an alcohol ?

Answer 2

R-OH

Question 3

Explain how alcohols are classified ?

Answer 3

Question 4

How are the carbons in an alcohol numbered ?

Answer 4

The alcohols are always numbered to give the carbon with the hydroxy (OH) group the lowest number

Question 5

What is the prefix for a 5 carbon chain ?

Answer 5

Pent

Question 6

What is the prefix for a 6 carbon chain ?

Answer 6

Hex

Question 7

What is the prefix for a 3 carbon chain ?

Answer 7

Prop

Question 8

When oxygen is bonded R-OH, how many lone E- pairs does it have ?

Answer 8

2 lone pairs

Question 9

Explai hydrogen bonding in alcohols ?

Answer 9

The partially positive hydrogen in one molecule is attracted to the partially negative oxygen in another molecule.

Question 10

Explain why methanol has a lower boiling point than water ?

Answer 10

2 partially positive hydrogens in each H20 are available to hydrogen bond, but only 1 partially positive hydrogen in an alcohol (due to R groups)m which means that the hydrogen bonding is easier to break.

Question 11

Explain the boiling point and solubility trends of alcohols ?

Answer 11

• Higher boiling points and greater solubility tha comparable alkanes, alkenes, aldehydes, ketones and alkyl halides
• This is due to greater polarity and hydrogen bonding

Question 12

Explain the pH trend of alcohols ?

Answer 12

• Decreases with increasing number of attached carbons
• Primary > Secondary > tertiary

Question 13

What is the name of an alcohols conjugate base ?

Answer 13

An alkoxide

Question 14

Are alcohols basic or acidic ?

Answer 14

They are weak acids

(weaker than water)

Question 15

What does miscible mean ?

Give an example

Answer 15

Soluble in water in any proportion

e.g. Methanol is miscible in water, butanol is slightly soluble but not miscible

Question 16

Why do alcohols become less soluble as the length of the Carbon chain (R Group) increases ?

Answer 16

The R group is non-polar, thus inhibiting the molecules ability to hydrogen bond.

Question 17

Describe the acidity and basicity of alcohols

Answer 17

Alcohols can dissociate into a proton and its conjugate alkoxide.

As weak acids, alcohols act as proton donors:

ROH + H₂O → RO ^- + H₃O⁺

As weak bases, alcohols act as proton acceptors:

ROH + HX → ROH₂⁺ + X^-

Question 18

Describe the effect of substituents on alcohol acidity

Answer 18

The more easily the alkoxide ion is accessible to H₂O, the easier it is stabilised through solution, therefore the more its formation is favoured leading to greater acidity.

Question 19

Describe the influence of inducting effects on alcohol acidity ?

Answer 19

• E^- withdrawing groups stabilise an alkoxide ion by spreading out the charge, therefore making the alcohol more acidic.
• E^- donating groups destabilise the an alkoxide anion, making the alcohol less acidic.

Question 20

Descibe the difference in boiling point between ethane_(g) and ethanol_(l) ?

Answer 20

• Ethane has a low boiling point as only very weak London (VDW) forces are holding the molecule together.
• Ethanol has a higher boiling point due to hydrogen bonding formation

Question 21

Describe an alcohols reaction with a base ?

Answer 21

• Alcohols are only weak acids, therefore they do not react with weak bases
• Alcohols react with strong bases such as NaH or Na or K metals

Question 22

Explain the 3 ways alcohols can be synthesised ?

Answer 22

1. Hydration of Alkenes
2. Reduction of Carbonyl compounds
3. Addition reactions with Grignard reagents

Question 23

What Carbonyl compounds can be reduced to give alcohols ?

Answer 23

• Aldehydes
• Ketones
• Carboxylic acids
• Esters

Question 24

What does LiAlH₄ reduce aldehydes and ketones to ?

Answer 24

Aldehydes ⇒ 1º Alcohols

Ketones ⇒ 2º Alcohols

Esters ⇒ 3º Alcohols

Question 25

Name 5 important reactions of Alcohols ?

Answer 25

1. Dehydration 2. Oxidation-reduction 3. Substitution 4. Elimination 5. Conversion to Alkyl Halides

Question 26

What do dehydration reactions of alcohols produce ?

Answer 26

Dehydration = loss of water, therefore alkenes are produced

Question 27

What does 'R' represent in organic chemistry ?

Answer 27

Denote attached hydrogen or hydrocarbon side chains of any length

Question 28

Describe Oxidation

Answer 28

The increasing of oxygen or decreasing of hydrogen **_OIL_** **O**xidation **I**s **L**oss

Question 29

Describe reduction

Answer 29

The decreasing of oxygen or increasing of hydrogen **_RIG_** **R**eduction **I**s **G**ain

Question 30

Under mild and abrasive conditions, what are primary alcohols oxidised to ?

Answer 30

Mild ⇒ Aldehyde Abrasive ⇒ Carboxylic Acid

Question 31

What are secondary alcohols oxidised to form ?

Answer 31

Ketones

Question 32

Explain oxidation of tertiary alcohols

Answer 32

Tertiary alcohols are very difficult to oxidise, they are unaffected by acidic conditions.

Question 33

What is meant by the term substitution reaction ?

Answer 33

One atom or group is replaced by another. E.g. In alcohols, the OH group can be substituted by a halide (Usually Cl or Br)

Question 34

Describe the term 'Sn1 Reaction'

Answer 34

* 1st order, or macromolecular (nucleophilic substitution) * The transition state involves a carbocation, the formation of which is the rate determining step.

Question 35

What Alcohol substitutions usually proceed by Sn1 ?

Answer 35

Reactions involving Benzyl groups, alkyl groups, tertiary and secondary alcohols.

Question 36

Name 3 important features of an Sn1 reaction ?

Answer 36

1. The reaction is 1st order (rate depends on concentration of 1 compound) 2. There is racemization of configuration when a chiral molecule is involved 3. A stable carbonium ion should be formed

Question 37

What is an Sn2 reaction ?

Answer 37

2nd order or bimolecular nucleophilic substitution

Question 38

Name 3 important points regarding Sn2 reactions ?

Answer 38

* The reaction rate is second order overall (rate depends on concentration of 2 compounds) * The nucelophile adds to the alkyl group at the opposite side to the ligand * Large or bulky groups near or at the site may hinder or retard a reaction (Steric hindrance)

Question 39

Explain steric hindrance

Answer 39

* Large or bulky groups near or at the reacting sites hinder or retard the rate of Sn2 reactions

Question 40

What reactions involving alcohols proceed via the Sn2 mechanism ?

Answer 40

Substitution reactions for Methanol and other primary alcohols

Question 41

Explain good leaving groups in organic chemistry ?

Answer 41

Good leaving groups have strong conjugate acids E.g H₂O because H₃O⁺ is a strong acid.

Question 42

What is an elimination reaction ?

Answer 42

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

Question 43

Describe E1 reactions ?

Answer 43

* The rate depends on the concentration of one compound * Often occurs as minor products alongside Sn2 reactions

Question 44

What would happen if you double the concentration of both reactants in an Sn2 reaction ?

Answer 44

In a second order reaction, the rate is proportional to the concentration of both reactants, therefore if both the concentrations were doubled, the rate would increase four-fold.