L12 worksheet Ans

CHEM1611 Worksheet Answers – Lecture 12 1. Indicate with an arrow the trend in acid strength and base strength (below) and with a √ or X which of the following acid – base reactions will occur. Base Acid NH2– OH– HCO3– (pKa = 25) (pKa = 15.7) (pKa = 6.35) ✓
✓
✓
✓
✓
✗
✓
✗
✗
Carboxylic acid (pKa ~ 5) Phenol (pKa ~ 10) Alcohol (pKa ~ 17) 2. Complete the table by drawing the correct form of the compound at the indicated pH. Compound Benzoic acid (pKa = 4.19) is found in mouth wash preparations. Structure H
O
C
C
pH = 7.0 C
C
C
C
O
OH
O
H
pH = 2.0 O
O
H
C
O
H
OH
H
Imidazole (pKa = 6.95) is a proton transfer catalyst when attached to proteins. H
H
H
N
N
H
C
~50%
N
N
C
H
N
OH
O
O
4
NH 3
NH 3
NH 3
NH 2
(H 2C) 4
OH
O
pKa=8.95
pKa=10.53
H
N
O
O
NH 2
~50%
O
pKa=2.18
NH 2
N
C
CH
N
N
H
H
H
H
N
C
Lysine, an essential amino acid pH = 9.0 4
4
NH 3
NH 3
3. Taken from the 2015 exam paper. Glycine, H2NCH2COOH, is the simplest of all naturally occurring amino acids. The pKa of the acid group is 2.35 and the pKa of the conjugate acid of the amino group is 9.78. Draw a Lewis structure of glycine that indicates the charges on the molecule at the physiological pH of 7.4. H
H
N
H
H
O
C
C
H
O
H
N
H
H
H
O
C
C
H
O
Comment on the lengths of the two carbon – oxygen bonds in your Lewis structure. Two resonance structures indicates they must be identical in length to one another. Complete the table by indicating the hybridization states and molecular geometry associated with the nitrogen and two carbon atoms in glycine. N C (alkyl) C (carboxylic) Hybridisation of sp3 sp3 sp2 atom Molecular geometry Trigonal pyramid Tetrahedron Trigonal planar around atom Propionic acid, CH3CH2COOH, has a similar molar mass to glycine and readily forms hydrogen bonded dimers (a pair of molecules) in liquid form. Draw a pair of propionic acid molecules showing the hydrogen bonding between them. O
H 3CH2C
H
O
C
C
O
H
CH2CH 3
O
The melting point of propionic acid is —20.7 °C and melting point of glycine is 292 °C. Suggest a reason why these two molecules have such different melting points. The charges on the glycine result in strong electrostatic (ionic) attraction between the molecules and a high mp. The charges are absent in propionic acid. 4. Electrophoresis is a technique in which an electrical potential is used to separate compounds based on their overall charge and is conducted in a buffered solution. At a pH = 8.0 will the following amino acids more towards the positive electrode, negative electrode or remain ~unmoved? Amino acid pKa (-‐COOH) pKa (-‐NH3+) pKa (sidechain) Movement? Alanine 2.34 9.87 -‐ ~unmoved Arginine 1.82 8.99 12.48 (-‐NH3+) Towards negative Aspartic acid 1.99 10.00 3.90 (-‐COOH) Towards positive

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