3.4. Covalent bonding and coordinate (dative covalent) bonding
A subsection of Chemistry, 9701, through 3. Chemical bonding
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Some metals are essential to biochemical processes. Complete the following table naming one metal in each case. biochemical process metal haemoglobin in oxygen transport transmission of nerve impulses enzyme cofactor Enzymes are a special type of protein molecule that catalyse biochemical reactions. Explain briefly the mechanism by which an enzyme breaks down a substrate molecule. Disulfide bonds play an important role in the stability of some proteins such as the keratin in human hair. The amino acid involved in the formation of a disulfide bond is cysteine, H2NCH(CH2SH)CO2H. At which level of protein structure (primary, secondary, tertiary) are disulfide bonds formed? Use a functional group in cysteine to show how disulfide bonds are formed. What type of chemical reaction is this? The NMR spectrum of cysteine, H2NCH(CH2SH)CO2H, shows five absorptions. After shaking a solution of cysteine with a few drops of D2O, the NMR spectrum shows only two absorptions, E and F, shown below. E F Identify the two types of protons responsible for the absorptions E and F. E F State and explain the splitting patterns of the absorptions E and F. E F
9701_w15_qp_41
THEORY
2015
Paper 4, Variant 1
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Some metals are essential to biochemical processes. Complete the following table naming one metal in each case. biochemical process metal haemoglobin in oxygen transport transmission of nerve impulses enzyme cofactor Enzymes are a special type of protein molecule that catalyse biochemical reactions. Explain briefly the mechanism by which an enzyme breaks down a substrate molecule. Disulfide bonds play an important role in the stability of some proteins such as the keratin in human hair. The amino acid involved in the formation of a disulfide bond is cysteine, H2NCH(CH2SH)CO2H. At which level of protein structure (primary, secondary, tertiary) are disulfide bonds formed? Use a functional group in cysteine to show how disulfide bonds are formed. What type of chemical reaction is this? The NMR spectrum of cysteine, H2NCH(CH2SH)CO2H, shows five absorptions. After shaking a solution of cysteine with a few drops of D2O, the NMR spectrum shows only two absorptions, E and F, shown below. E F Identify the two types of protons responsible for the absorptions E and F. E F State and explain the splitting patterns of the absorptions E and F. E F
9701_w15_qp_42
THEORY
2015
Paper 4, Variant 2
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G belongs to a group of compounds called ethers. H H C H H C H H C H H C H H O G When G, C4H10O, is heated, thermal decomposition occurs. C4H10O→ C2H6+ CO+ CH4∆H = –7.00 kJ mol–1 The atoms in a molecule of CO are held together by a triple covalent bond. One of these bonds is a coordinate (dative covalent) bond. Draw a dot-and-cross diagram to show the arrangement of outer electrons in a CO molecule. Use ● to represent electrons from an oxygen atom. Use × to represent electrons from a carbon atom.  Calculate the bond energy of C≡O using the bond energy values in Table3.1 and the enthalpy change, ∆H, for the thermal decomposition of G. Show your working. Table 3.1 bond bond energy / kJ mol–1 C–C C–O (in G) C–H  bond energy (C≡O) = kJ mol–1  When G, C4H10O, is heated in a sealed container, an equilibrium mixture is produced. C4H10OC2H6+ CO+ CH4Complete the expression for the equilibrium constant, Kc, for this reaction. State the units of Kc. Kc =  units =  Thermal decomposition of G in the presence of I2 affects the activation energy, Ea, for the reaction. Table3.2 shows Ea for the thermal decomposition of G with and without I2. Table 3.2 reaction Ea (with I2) / kJ mol–1 Ea / kJ mol–1 C4H10O→ C2H6+ CO+ CH4143 State what effect adding I2 to the reaction mixture has on the value of Kc. Explain your answer.  the Boltzmann distribution of energies for molecules of G at constant temperature, T °C. Sketch, on Fig.3.2, the Boltzmann distribution of energies for molecules of G at a higher temperature, (T+100) °C. number of molecules energy Ea  The functional group in G is an oxygen atom bonded to two carbon atoms. H H C H H C H H C H H C H H O G G, H and J are structural isomers with molecular formula C4H10O. H and J are straight chain molecules. Table3.3 shows the boiling points and reactions of G, H and J when heated under reflux with excess acidified K2Cr2O7. Table 3.3 G H J boiling point / °C heat under reflux with excess acidified K2Cr2O7 remains orange orange to green orange to green Identify the type of structural isomerism shown between G and H using the information in Table3.3. Identify the type of structural isomerism shown between H and J using the information in Table3.3. Draw a possible structure for H and for J. State the systematic name for each structure. H name J name  K has molecular formula C3H6O. When K is added to 2,4-dinitrophenylhydrazine, an orange precipitate forms. When K is warmed with Tollens’ reagent, a silver mirror forms. Draw the displayed formula of K. K  
9701_s22_qp_23
THEORY
2022
Paper 2, Variant 3
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Questions Discovered
93