Org 6 - Alcohols Flashcards
What is an alcohol ?
Any organic compound in which a hydroxyl group (OH) is bound to a carbon atom of an Alkyl or substituted alkyl group.
What is the general formula of an alcohol ?
R-OH
Explain how alcohols are classified ?
How are the carbons in an alcohol numbered ?
The alcohols are always numbered to give the carbon with the hydroxy (OH) group the lowest number
What is the prefix for a 5 carbon chain ?
Pent
What is the prefix for a 6 carbon chain ?
Hex
What is the prefix for a 3 carbon chain ?
Prop
When oxygen is bonded R-OH, how many lone E- pairs does it have ?
2 lone pairs
Explai hydrogen bonding in alcohols ?
The partially positive hydrogen in one molecule is attracted to the partially negative oxygen in another molecule.
Explain why methanol has a lower boiling point than water ?
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.
Explain the boiling point and solubility trends of alcohols ?
- Higher boiling points and greater solubility tha comparable alkanes, alkenes, aldehydes, ketones and alkyl halides
- This is due to greater polarity and hydrogen bonding
Explain the pH trend of alcohols ?
- Decreases with increasing number of attached carbons
- Primary > Secondary > tertiary
What is the name of an alcohols conjugate base ?
An alkoxide
Are alcohols basic or acidic ?
They are weak acids
(weaker than water)
What does miscible mean ?
Give an example
Soluble in water in any proportion
e.g. Methanol is miscible in water, butanol is slightly soluble but not miscible
Why do alcohols become less soluble as the length of the Carbon chain (R Group) increases ?
The R group is non-polar, thus inhibiting the molecules ability to hydrogen bond.
Describe the acidity and basicity of alcohols
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-
Describe the effect of substituents on alcohol acidity
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.
Describe the influence of inducting effects on alcohol acidity ?
- 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.
Descibe the difference in boiling point between ethane(g) and ethanol(l) ?
- 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
Describe an alcohols reaction with a base ?
- 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
Explain the 3 ways alcohols can be synthesised ?
- Hydration of Alkenes
- Reduction of Carbonyl compounds
- Addition reactions with Grignard reagents
What Carbonyl compounds can be reduced to give alcohols ?
- Aldehydes
- Ketones
- Carboxylic acids
- Esters
What does LiAlH4 reduce aldehydes and ketones to ?
Aldehydes ⇒ 1º Alcohols
Ketones ⇒ 2º Alcohols
Esters ⇒ 3º Alcohols
Name 5 important reactions of Alcohols ?
- Dehydration
- Oxidation-reduction
- Substitution
- Elimination
- Conversion to Alkyl Halides
What do dehydration reactions of alcohols produce ?
Dehydration = loss of water, therefore alkenes are produced
What does ‘R’ represent in organic chemistry ?
Denote attached hydrogen or hydrocarbon side chains of any length
Describe Oxidation
The increasing of oxygen or decreasing of hydrogen
OIL
Oxidation Is Loss
Describe reduction
The decreasing of oxygen or increasing of hydrogen
RIG
Reduction Is Gain
Under mild and abrasive conditions, what are primary alcohols oxidised to ?
Mild ⇒ Aldehyde
Abrasive ⇒ Carboxylic Acid
What are secondary alcohols oxidised to form ?
Ketones
Explain oxidation of tertiary alcohols
Tertiary alcohols are very difficult to oxidise, they are unaffected by acidic conditions.
What is meant by the term substitution reaction ?
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)
Describe the term ‘Sn1 Reaction’
- 1st order, or macromolecular (nucleophilic substitution)
- The transition state involves a carbocation, the formation of which is the rate determining step.
What Alcohol substitutions usually proceed by Sn1 ?
Reactions involving Benzyl groups, alkyl groups, tertiary and secondary alcohols.
Name 3 important features of an Sn1 reaction ?
- The reaction is 1st order (rate depends on concentration of 1 compound)
- There is racemization of configuration when a chiral molecule is involved
- A stable carbonium ion should be formed
What is an Sn2 reaction ?
2nd order or bimolecular nucleophilic substitution
Name 3 important points regarding Sn2 reactions ?
- 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)
Explain steric hindrance
- Large or bulky groups near or at the reacting sites hinder or retard the rate of Sn2 reactions
What reactions involving alcohols proceed via the Sn2 mechanism ?
Substitution reactions for Methanol and other primary alcohols
Explain good leaving groups in organic chemistry ?
Good leaving groups have strong conjugate acids
E.g H2O because H3O+ is a strong acid.
What is an elimination reaction ?
When an atom or group of atoms is removed (eliminated) from adjacent carbons leaving a multiple bond.
Describe E1 reactions ?
- The rate depends on the concentration of one compound
- Often occurs as minor products alongside Sn2 reactions
What would happen if you double the concentration of both reactants in an Sn2 reaction ?
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.
