Edexcel A Level Chemistry:复习笔记7.1.1 Chirality

Chirality

 

Optical isomers

  • A carbon atom that has four different atoms or groups of atoms attached to it is called a chiral carbon or chiral centre
    • Chira comes from a Greek word meaning hand, so we talk about these molecules having a handedness

     

  • The carbon atom is described as being asymmetric, i.e. there is no plane of symmetry in the molecule
  • Compounds with one chiral centre (chiral molecules) exist as two optical isomers, also known as enantiomers
  • Just like the left hand cannot be superimposed on the right hand, enantiomers are non-superimposable
    • Enantiomers are mirror images of each other

     

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Enantiomers-and-Chiral-Centre

A molecule has a chiral centre when the carbon atom is bonded to four different atoms or group of atoms; this gives rises to enantiomers

Exam Tip

When drawing optical isomers, always draw mirror images including wedge and dashed bonds

20-3-3-drawing-optical-isomers-exam-tip

The Nature of a Racemic Mixture

Properties of optical isomers

  • The chemical properties of optical isomers are generally identical, with one exception
  • Optical isomers interact with biological sensors in different ways
  • For example, one enantiomer of carvone smells of spearmint, while the other smells of caraway

20-3-3-carvone-optical-isomers

                         Carvone optical isomers have distinctive smells

 

  • Optical isomers have identical physical properties, with one exception
    • Isomers differ in their ability to rotate the plane of polarised light

     

7.1-Organic-Chemistry-Unpolarised-Light

When unpolarised light is passed through a polariser, the light becomes polarised as the waves will vibrate in one plane only

 

  • The major difference between the two enantiomers is:
    • One enantiomer rotates plane polarised light in a clockwise manner and the other in an anticlockwise fashion
    • A common way to differentiate the isomers is to use (+) and (-), but there are other systems using d and l, D and L, or R and S

     

  • The rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance
    • For example, pass plane polarised light through a sample containing one of the two optical isomers of a single substance
    • Depending on which isomer the sample contains, the plane of polarised light will be rotated either clockwise or anti-clockwise by a fixed number of degrees

     

7.1-Organic-Chemistry-Effect-of-Optical-Isomers-on-Plane-of-Polarised-LightEffect-of-Optical-Isomers-on-Plane-of-Polarised-Light

Each enantiomer rotates the plane of polarised light in a different direction

 

  • A racemic mixture (or racemate) is a mixture containing equal amounts of each enantiomer
  • One enantiomer rotates light clockwise, the other rotates light anticlockwise
  • A racemic mixture is optically inactive as the enantiomers will cancel out each others effect

This means that the plane of polarised light will not change20-3-3-racemic-mixture

Racemic mixtures are optically inactive

Racemic mixtures and drugs

  • In the pharmaceutical industry, it is much easier to produce synthetic drugs that are racemic mixtures than producing one enantiomer of the drug
  • Around 56% of all drugs in use are chiral and of those 88% are sold as racemic mixtures
  • Separating the enantiomers gives a compound that is described as enantiopure, it contains only one enantiomer
  • This separation process is very expensive and time consuming, so for many drugs it is not worthwhile, even though only half the of the drug is pharmacologically active
  • For example, the pain reliever ibuprofen is sold as a racemic mixture

7.1.3-Ibuprofen

The structure of ibuprofen showing the chiral carbon that is responsible for the racemic mixture produced in the synthesis of the drug

Optical Activity & Mechanisms

  • Optical activity can be used to suggest the mechanism of a chemical reaction
  • This is particularly the case for nucleophilic substitution
    • Nucleophilic substitution can occur via an SN1 or SN2 mechanism

SN1 mechanism

  • The SN1 mechanism is a two-step reaction
    • In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X- ion
    • This leaves a trigonal planar, tertiary carbocation
    • In the second step, the planar, tertiary carbocation is attacked by the nucleophile
    • The nucleophile is able to attack from either side of the planar carbocation, which results in the formation of a racemic mixture
  • Therefore, a reaction with an SN1 mechanism will produce a racemic mixturesn1-optical-isomers-mechanism
    SN1 Optical Isomers Mechanism

SN2 mechanism

  • The SN2 mechanism is a one-step reaction
    • The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond
    • At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond
    • The halogen leaves the halogenoalkane as an X- ion

     

  • For example, the nucleophilic substitution of bromoethane by hydroxide ions to form ethanol

20.1-The-SN2-mechanism-of-bromoethane-with-hydroxide-causing-an-inversion-of-configuration

The SN2 mechanism of bromoethane with hydroxide causing an inversion of configuration

 

  • The bromine atom of the bromoethane molecule causes steric hindrance
  • This means that the hydroxide ion nucleophile can only attack from the opposite side of the C-Br bond
    • Attack from the same side as the bromine atom is sometimes called frontal attack
    • While attack from the opposite side is sometimes called backside or rear-side attack

     

  • As the C-OH bond forms, the C-Br bond breaks causing the bromine atom to leave as a bromide ion
    • As a result of this, the molecule has undergone an inversion of configuration
    • The common comparison for this is an umbrella turning inside out in the wind

     

Inversion-of-configuration-%E2%80%93-umbrella-analogy

Inversion of configuration - umbrella analogy

 

  • Therefore, if a reaction with an SN2 mechanism starts with an enantiopure reactant then an enantiopure products will be formed

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