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What are amines?

Amines are a class of compounds related to ammonia, NH3. They are organic chemicals where one or more of the hydrogen atoms on ammonia have been replaced by alkyl chains. 


What are the three types of amine?

  • Primary, 1o - one hydrogen atom has been substituted. The structural formula can be summarised as RNH2 where R is the alkyl chain.
  • Secondary, 2o - two hydrogen atoms have been substituted. The structural formula can be summarised as RNH2 where R and R' are the alkyl chains, which may be the same or different. 
  • Tertiary, 3o - all three hydrogen atoms have been substituted. The structural formula can be summarised as RNR'R'' where R, R', and R'' can be the same or different hydrocarbon groups. 


How do you name amines? 

When naming amines, the suffix is always amine. Then alkyl chains must be determined and used to generate the root and prefix of the name.


Describe the naming process for primary amines. 

Primary → determine root by longest hydrocarbon chain and any prefixes for other groups. Finally, add the suffix amine. 


Describe how you name a secondary amine. 

For secondary amines - determine the root by naming two alkyl chains. Add any prefixes for additional groups and write them in alphabetical order. Finally, add the suffix 'amine'. As the alkyl groups are attached to the nitrogen atom, secondary amines are often called 'N-substituted' and this is given as a prefix to the name.  


Describe how you name a tertiary amine. 

For tertiary amines - determine the root by naming the three alkyl chains. If there is more than one of the same group add the appropriate prefix; di or tri-


Define a lewis base.

A lewis base has a lone pair of electrons for donation - ammonia and amines have a lone pair of electrons on the nitrogen atom, making them Lewis bases.


Define a brønstead-Lowry base. 

A Brønstead-lowry base is a proton acceptor - ammonia and amines can accept protons on the nitrogen atom and are therefore also Brønsted-Lowry bases. 


Why are amines and ammonia both weak bases? 

Amines and ammonia are both weak bases. Using the Brønsted-Lowry model, when an amine reacts with an acid, it accepts a proton. A covalent bond is formed by the nitrogen atom donating its lone pair of electrons to the proton to form a dative covalent (or coordinate) bond. 


Describe the reactions of amines with dilute inorganic acids. 

When HCl(aq) reacts with a base, a chloride salt and water are produced. When primary amines react with this acid, an alkylammonium salt is made. This is formed by the proton in the acid bein replaced by an alkylammonium ion. 

CH3CH2NH3 + HCl → CH3CH2NH2 + Cl-


What is an alkylammonium salt?

An alkylammonium salt is a compound where the hydrogen(s) on an ammonium ion have been substituted by alkyl chains. 


Why are secondary and tertiary amines also bases?

Secondary and tertiary amines are also bases as they have a lone pair of electrons on the nitrogen atom in their functional group. This also allows them to react with acids and form salts.  


Draw the structural formula chemical equation for the reaction of ethylamine with dilute hydrochloric acid. 


How will other strong inorganic acids such as nitric acid react with amines? 

Other strong inorganic acids, such as nitric and sulphuric acid will react in a similar way to make a salt. 

When nitric acid is used instead of hydrochloric acid the nitrate ion NO3- is simply substituted for the chloride ion Cl- in the balanced chemical equation.

CH3CH2NH2 + HNO3 + HNO3 → CH3CH2NH3+NO3-


How will other strong inorganic acids such as sulphuric acid react with amines? 

When sulphuric acid is used instead of hydrochloric acid the balanced chemical equation is more complex. 

2CH3CH2NH2 + H2SO4 → [CH3CH2NH3+]2SO42-


Name the salts formed from the reaction of amines with HNO3, H2SO4, and HCl. 

  1. HCl = alkylammonium chloride
  2. HNO3 = alkylammonium nitrate
  3. Sulphuric acid = alkylammonium sulphate 


How may the molecular ratio of amine to acid change in the reaction between an amine and an acid. 

When an amine reacts with sulphuric acid two molecules of amine are required for every molecule of sulphuric acid. This is because the alkylammonium sulphate produced contains two alkylammonium ions for every one sulphate ion. Alkylammonium ions have a single positive charge whilst a sulphate ion has a double negative charge. 


Describe the preparation of aliphatic amines.

Using a sealed test tube, a haloalkane, ammonia and ethanol are heated together to make an amine. Reflux cannot be used as the ammonia is so volatile it would escape out of the condenser rather than react.

A haloalkane, such as 1-chloropropane, will undergo nucleophilic substitution in a two-stage process to form a primary amine. 


Give the two stages of forming the aliphatic amine, 1-chloropropane.

  • Stage 1: The ammonia reacts with the haloalkane to make an ammonium salt.


  • Stage 2: An additional ammonia molecule reacts to form the propylamine product and ammonium chloride salt. 

CH3CH2CH2NH3Cl + NH3 ⇔ CH3CH2CH2NH2 + NH4Cl 

(as this is a reversible reaction the addition of excess ammonia will drive the reaction to the right and increase the yield of the desired primary amine product)


During the preparation of aliphatic amines, additional substitution of the hydrogen atoms on the nitrogen atom can occur. What products could be made in this reaction?

  • In this reaction, initially, N-dipropylamine, a secondary amine, is made. 

CH3CH2CH2Cl + (CH3CH2CH2)2NH ⇔ (CH3CH2CH2)3N+HCl

  • Further substitution would produce N-tripropylamine, a tertiary amine.

CH3CH2CH2Cl + (CH3CH2CH2)2NH2 ⇔ (CH3CH2CH2)3N + HCl 

  • The final stage is a quaternary ammonium salt, where each hydrogen on the ammonium ion has been replaced with an alkyl chain.

CH3CH2CH2Cl + (CH3CH2CH2)3N ⇔ (CH3CH2CH2)4N+Cl-

  • These further substitution reactions can occur because amines have lone pairs of electrons on the nitrogen atom that can act as a nucleophile. 


How does the ratio of reactants used in the preparation of an aliphatic amine affect the type of salt produced? 

When preparing amines by this method there is always a mixture of the products produced. As the second stage of the mechanism is an equilibrium reaction, using excess ammonia favours the primary amine and excess haloalkane favours the quaternary ammonium salt. 


How are aromatic amines prepared? 

  • Nitroarenes, such as nitrobenzene, can be reduced to produce an amine. The reducing agent is made in situ by using a mixture of tin and concentrated HCl. The reaction occurs at 100oC.
  • After about half an hour, a strong alkali, such as sodium hydroxide, is added. This undergoes a neutralisation reaction to remove the excess hydrochloric acid and produce the amine. Separating the aromatic amine is a multi-stage process tat includes steam distillation, solvent extraction and further distillation. 


Describe the class of compounds, amino acids. 

Amino acids are a class of compounds with two functional groups: a carboxylic acid, -COOH, and an amine group, -NH2.  When both functional groups are attached to the same carbon atom, the compound is called a σ-amino acid. This group of organic chemicals has the general formula RCH(NH2​)COOH. 


Why are proteins a useful molecule?

Proteins are an essential nutrient for all animals as they are used to make, among other things, hormones, muscles and enzymes. All proteins are polymer chains made up of amino acid monomers bonded together. In human biochemistry there are around 20 σ-amino acids. 


Why are amino acids amphoteric? 

The carboxylic acid functional group is a weak acid that will partially ionise in water. The nitrogen atom on the amine group has a lone pair of electrons and can act as a base. This means that amino acids are amphoteric (they can act can act as an acid and a base), as their carboxylic acid group can react with bases and their amine group can react with acids. 


How do σ-amino acids form zwitterions? 

σ-amino acids have the two functional groups, -COOH and -NH2, attached to the same carbon atom. σ-amino acids can form zwitterions, where their two functional groups exchange a proton and make an internal salt. The carboxylic acid donates a proton to the amino group. As the two charges cancel each other out, the resultant molecule has no overall charge. 


Describe how an amino acid is affected by changes in the pH. 

The isoelectric point is when there is no net electrical charge due to each zwitterion having an internal balance of charge. By changing the pH you alter the amino acid so that only one of its functional groups is charged. This means at a low pH, where there is a lot of H+, the carboxylic acid functional group becomes -COOH and only the amine group is charged. However, at high pH, the amine group becomes -NH2 and only the carboxylic acid group is charged. 


Draw a zwitterion in a low and high pH. 


The carboxylic acid functional group is a weak acid and partially ionises in solution. This functional group will undergo all the common reactions of an organic acid what reactants? 

  • Metal oxides
  • Alkalis
  • Carbonates
  • Alcohol


 Describe the reaction between metal oxides and the carboxylic acid functional group of amino acids.

Metal oxides - metal oxides are bases, so a neutralisation reaction occurs. The hydrogen atom on the carboxylic acid group is exchanged for a metal ion to produce a metal salt. Water is also produced in this reaction.