3.5 Optical Isomerism
- Optical isomerism arises because of the different ways in which you can arrange four groups around a carbon atom- a type of stereoisomerism
- Can only occur when four different atoms/groups are attached to a central carbon atom.
- The optical isomers or enantiomers are non-superimposable, mirror images of each other
- Molecules that are not superimposable on their mirror images are called chiral molecules. A carbon atom that is bonded to four different groups is called a chiral centre.
- Proteins are enantiomers, and are classed as L-enantiomers or D-enantiomers, depending on how they obey the CORN rule.
- Stand your model/arrange your diagram with the H atom pointing upwards and look down the H atom. The L-amino acid will have the CarbOxylic acid, R group and amiNe (CO R N) in a clockwise direction
- Enantiomers will behave identically in test tube reactions and have the same physical properties, but will behave differently in the presence of other chiral molecules (such as enzymes and those in taste buds and up the nose)
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10.3 The Effect of Concentration on Rate
- Knowing how concentration affects rate tells us about the mechanisms (way a reaction occurs). Rate of reaction= the rate at which reactants are converted into products
- Measuring R of R- a plot of a property against time gives a progress curve
- 1. Measuring volumes of gas evolved
- 2. Measuring mass changes
- 3. pH measurement
- 4. Colorimetry
- 5. Chemical analysis and titration
- Rate of reaction changes as the reactants get used up
- The half-life of a reaction is the time taken for a concentration of a reactant to fall by half or the original value. A reaction with a constant half-life is first order with respect to that reactant
- The rate equation shows how rate of reaction varies with the concentration of reactants. It can only be predicted from the chemical equations of the individual steps, and must be found experimentally
- Many reactions occur in stages- the slowest step is the rate-determining step.
- For reaction of A and B, the rate equation is rate = k [A]^m [B]^n, where k is the rate constant and m and n are the orders of the reaction for each reactant. The overall order of reaction is m + n
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10.3 The Effect of Concentration on Rate 2
- It is important to make measurements at a constant temp and only investigate one reactant at a time. Other concentrations must be kept constant
- To find the rate of reaction with respect to reactant A, to R of R is found at different concentrations of A, by drawing tangents on the progress curve or finding the initial rate of reaction for different reaction mixtures
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10.4 Enzymes and Rate
- Enzymes are specific because they have a precise tertiary structure which exactly matches the structure of the substrate.
- Enzymes have a cleft in their structure which is tailored for substrate molecules. The cleft contains chemical groups for the substrate to bind and react with. This is the active site.
- The bonds binding an enzyme and substrate must be weak so the binding can be easily reversed when the products leave the active site (usually H bonds or ionic interactions)
- When substrate concentration is low, not all enzyme active sites will have substrate molecules bound to them. R of R depends on how much substrate there is. Reaction rate is 1st order with respect to substrate concentration
- If substrate concentration is high, the reaction is zero order with respect to substrate concentration, as all enzyme active sites are occupied as [ES] is constant
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13.9 Amino Acids and Proteins
- Amino acids are bi-functional compounds- 2 functional groups. They have a proton-donating -COOH and a proton-accepting NH2 groups. The groups can react with each other to form zwitterions, with both positively charged and negatively charged groups.
- An aqueous solution of aa consists mainly of zwitterions. aa are soluble in water as they are effectively ionic
- Adding small quantities of acid or alkali to an aa solution causes little change in pH, as the zwitterions neutralise the effect of the addition. Solutions which can withstand additions of acid and alkali are buffer solutions.
- When an -NH2 reacts with a -COOH, a secondary amide group is formed. When two aa join in this way with a condensation reaction, the secondary amide group formed is called a peptide link. Polypeptides can be formed in this way.
- The order of aa in a protein is the primary structure.
- Polypeptide chains are folded, are held together with ID-ID bonds between non-polar side chains, H bonds between polar side chains, ionic bonds between ionisable side chains, and covalent bonds
- Secondary structure- when the primary structure is coiled into a helix or stretched into stacked chains- hydrogen bonding. The structure is further folded into the tertiary structure, held by ID-ID, H bonds, ionic and covalent
- The peptide link can be hydrolysed through heating with mod conc acid or alkali
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15.9 Atom Economy and Enzymes
- Enzymes increase atom economy by reducing the number of steps in a reaction, and reduced energy requirements.
- Enzymes can also be reused and their large size makes them easy to separate from the products of the reaction
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