Synthesis 4-6 Flashcards
Why do alcohols have higher boiling points than other organic compounds?
Alcohols contain polar OH groups which allows them to form hydrogen bonds between molecules. Hydrogen bonds are the strongest Van der Waals forces so a lot of energy is needed to break them, giving alcohols higher boiling points.
Why does the solubility of alcohols decrease as the length of the hydrocarbon increases?
The lower alcohols (methanol, ethanol etc) are more miscible with water because they can hydrogen bond with water molecules. However, the higher alcohols (hexanol, heptanol etc) are insoluble in water because their long, non-polar hydrocarbon chains cancel out the hydrogen bonding of hydroxide groups.
How can alcohols be made?
1) Nucleophilic substitution reactions of monohaloalkanes - when a monohaloalkane is heated under reflux with aqueous sodium or potassium hydroxide the OH removes and replaces the halogen atom, forming an alcohol.
2) Acid-catalysed additon/hydration reaction of alkenes - when an alkene is reacted with water in the presence of sulfuric acid as a catalyst, a water molecule is added to the double bond, forming an alcohol.
3) Reduction reaction of aldehydes/ketones - when an aldehyde or ketone is reacted with lithium aluminium hydride dissolved in ether, 2 hydrogen atoms are added to the aldehyde/ketone (one reacts with the oxygen atom to form a OH group and the other reacts with the carbon atom attached to the oxygen groups).
Describe the reaction between an alcohol and a reactive metal such as sodium or potassium.
Alcohols can undergo a displacement/redox reaction with a reactive metal such as sodium or potassium to form an alkoxide. For example, sodium can react with ethanol to form sodium ethoxide. The hydrogen from the OH group is removed and replaced with the reactive metal atom.
Describe the reaction between an alcohol and aluminium oxide, concentrated sulfuric acid or concentrated phophoric acid.
When an alcohol undergoes a a dehydration/elimination reaction with aluminium oxide, conc sulfuric acid or conc phosphoric acid, and alkene is produced. The hydroxide group and an adjacent hydrogen atom is removed to produce a molecule of water and the two carbon atoms they were removed from form a double bond. This reaction can result in two alkenes being formed depending on which adjacent hydrogen atoms is removed.
Describe the reaction between an alcohol and a carboxylic acid or acid chloride.
An alcohol can undergo a condensation/esterification reaction with a carboxylic acid or an acid chloride to form an ester.
- The reaction between an alcohol and carboxylic acid is slow and requires a catalyst of concentrated sulfuric acid. The H from the OH group on the alcohol and the OH part of the carboxylic acid join together to form a water molceule and the alcohol and acid then join together to form an ester.
- The reaction between an alcohol and an acid chloride is much faster and no catalyst is required. The H from the OH group reacts with the chlorine atom from the acid chloride to form HCL.
What is an ether?
An ether is an alkane with an alkoxy group attached (a functional group - an alkyl group attached to an oxygen atom) and they have the general structure R-O-R, where R is alkyl groups which can either be the same or different.
How are ethers named?
1) The ‘yl’ is removed from the name of the alkyl group and replaced with ‘oxy’ (methyl becomes methoxy, ehtyl becomes ehtoxy etc).
2) The longest carbon chain is the parent hydrocarbon and is written after the allkoxy group.
3) The carbon number the alkoxy group is attached to must be stated first.
What are the physical properties of ethers?
- Ethers have lower boiling ponts than their isomeric alcohols because they are unable to hydrogen bond between other ether molecules. This is because the highly electronegative oxygen atom is not directly bonded to a hydrogen atom.
- Ether molecules are able to hydrogen bond with water which explains why smaller ethers with a low molecular mass are soluble in water. Larger ethers are insoluble in water as the non-polar hydrocarbon chain cancels out the effect of hydrogen bonding. Therefore, larger ethers are useful for extracting organic compounds from aqueous reaction mixtures.
- Ethers are volatile which allows them to be easily removed by distilation. They are also very flammable. Because they are polar, most organic compounds can easily absorb in them so they are often used as solvents.
How can ethers be prepared?
Ethers can be prepared by refluxing a haloalkane with an alkoxide in a nucleophilic substition reaction. For example, ethoxyethane is prepared by reacting chloroethane with potassium ethoxide in ethanol.
How can alkenes be prepared?
1) Dehydration of alcohols
2) Base-induced elimination of hydrogen halides from monohaloalkanes.
What compounds can alkenes undergo addition reactions with?
1) Hydrogen - This is known as hydrogenation and is catalysed by nickel or platinum. The hydrogens are added to the carbons with the double bond.
2) A halogen - This is known as halogenation and the the halogen atoms are added to the carbons with the double bond.
3) A hydrogen halide (e.g. H-Br) - This is known as hydrohalogenation and the hydrogen atom and halogen atom are added to the carbons with the double bond.
4) A water molecule - This is known as hydration and is catalysed by acids. The OH and H is added to the carbons with the double bond.
What is Markovnikov’s rule?
Markovnikov’s rule states that the main product of a reaction between an unsymmetrical alkene and a hydrogen halide or water, is the one in which the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogens attached to it.
Describe in detail the mechanism for a halogenation reaction between bromine and ethene.
This is a two-step reaction:
1) The bromine molecule (Br2) approaches the double bond which causes it to become polarised. The electron-rich double bond in ethene repels the electrons in the bromine molecule towards the one furthest away. This causes the bromine nearest ethene to become slightly positive. The electrophilic bromine atom attacks the nucleophilic double bond and breaks away from the other bromine atom which becomes a negative ion. This forms a cyclic ion intermediate.
2) The negative bromine ion now acts as a nucleophile and attacks the cyclic ion intermediate from the side opposite the other bromine atom.
Describe in detail the mechanism for a hydrohalogenaion reaction between hydrogen bromide and propene.
This is a two-step process:
1) The H-Br molecule is polarised (due to different electronegativities) and so is acting as an electrophile. The slightly positive hydrogen atom breaks away from the bromine atom and attacks the nucleophilic double bond in propene, forming an intermediate carbocation and a bromine ion.
2) The negative bromine ion acts as a nucleophille and can attack the carbocation from either side as the hydrogen atom is small.
Since the bromine ion can attack from either side, two products can be produced. The major product will be the one formed from the most stable carbocation intermediate.
