351 MT Fall 1999

Here is an post-mortem analysis / "how to" for the MT. The questions are split by the sections. At the start of each section are a few suggestions of what to look for or how to tackle the question type.

RELATIVE PROPERTIES:
Identify the controlling feature, which is not always as obvious as it may appear. Look for two pairs of similar systems to compare that have minimal differences in structure.

Qu1: C
The more electronegative atoms create the stronger dipoles. The length of the C-Cl bond also increases that dipole.

Qu2: B
The stronger the bonding interaction between the two C atoms the shorter the bond.

Qu3: E
Remember like dissolves like, so the hydrocarbon is soluble, the more polar groups there are, especially those that hydrogen bond, the less soluble it will be in hexane. A question about intermolecular forces.

Qu4: C
All staggered conformations so look for the gauche interaction of the two Me groups > anti Me groups and > all H system.

Qu5: C
Ring strain makes the heat of combustion more exothermic per -CH2-, so the smallest ring will have the most exothermic value, and thus cyclohexane > cyclobutane > cyclopropane.

Qu6: C
All O -ve systems, but resonance (note the presence of the adjacent pi system) allows it to be delocalised over three O atoms in ii, delocalised over two O atoms in i and localised on a single O in iii.

Qu7: A
You need to look at how many different groups are on each end of the system as this controls to number of different conformations that are possible. Since there are three different substituents in i (Br, Cl and H) each position of the -OH is unique, in ii there are only two (Br and H) so there are only two different positions of the -OH (gauche to two Br or gauche to H and Br.

Qu8: D
VSEPR and repulsion of lone pair - lone pair > lone pair - bonded pair > bonded pair - bonded pair.

LABORATORY:
Based on the general principles cover in the laboratory so far. Need to know the principles and details of the steps in the experiments.

Qu9: B
Melting points are not very sensitive to the applied pressure and are not corrected. It is boiling points that need to be corrected.

Qu10: B
Slow heating ensures an accurate reading on the thermometer.

Qu11: B
Sublimation is the reverse process, solid phase to the vapour phase.

Qu12: B
A Buchner funnel is the large ceramic funnel used for vacuum filtrations. A separatory funnel is used for extractions.

Qu13: B
The flow of water should be through the condenser. If the fractionating column is cooled, then no vapour will reach the condenser..

Qu14: B
If it were the same, then distillation would not be possible. The vapour will be richer in the more volatile component.

Qu15: A

Qu16: B
Melting points of impure materials are lower than those of pure materials.

Qu17: A

Qu18: B
Rf is defined as (distance traveled by sample) / (distance traveled by solvent front)

MOLECULAR PROPERTIES:
No real method here, really just do you know various aspects of molecular structure.

Qu19: A
C8 is attached to 3 electronegative O atoms (count -1 per bond) and a C atom (count 0), sum = -3, therefore the C8 is +3.

Qu20: D
C12 is attached to a single electronegative N atom (count -1), a C atom (count 0), and 2 H (count +1 each) sum = +1, therefore C12 is -1.

Qu21: E
C11 and C15 are sp3, all the others asked are sp2 as they are part of double bonds.

Qu22: B
O9 is sp2 as it is part of a carbonyl group.

Qu23: B
O10 is involved in resonance as so should be described as being sp2 hybridised.

Qu24: BE
There are both amines (N1 and N13) and an ester (C8,O9,O10,C11)

Qu25: C
Para means a 1,4- relationship, so since the amino is at C2 then para is C5

CONFORMATIONAL ANALYSIS:
The quickest method is to realise that the high energy points are eclipsed conformations (so B, E or AB) and the low energy are staggered (so A, C, D or AC). Note AD is not a representation of 2,2,3-trimethylpentane. A good idea to draw the structure first and label the C3-C4 bond. Then look at the position of the groups with respect to each other in the answers to identify the destabilising interactions.

Qu26: B
This is the highest energy eclipsed conformation, corresponding to the tBu and a Me interaction. If you get this one, then the rotation angle can also help out since now we know that the angle is that between the tBu bond and the Me group.

Qu27: A
This is the highest energy staggered conformation, corresponding to having a Me group gauche to the tBu and a Me.

Qu28: E
This is the second highest energy eclipsed conformation, corresponding to two Me groups eclipsed while the tBu is eclipsed with an H.

Qu29: D
This is the most stable staggered conformation. The tBu is gauche to 2 H atoms, there is only one gauche Me / Me pair.

Qu30: AB
The most stable eclipsed conformation, each of the alkyl groups only eclipsing with H atoms, so low interactions.

Qu31: AC
The second most stable staggered conformation with the tBu gauche to a Me and an H. Note that AC is actually the non-superimposable mirror image of C.


NOMENCLATURE:
Two types of questions here, however it is a good idea to use key descriptors like E, Z, cis or trans, numbering, functional group etc. to help screen the answers before getting too tied up in the words. For the first four, name the compound and then match it to the list, for the last four use the key descriptors and check each molecule in turn. So this section really depends on the nomenclature rules.

Qu32: C
Two methyl groups are on the same (upper) face of the cyclic structure, carbonyl group is C1.

Qu33: E
The C=C is E, FG = carboxylic acid, these two facts imply E.

Qu34: D
An isopropyl ester so C,D or E.... longest chain is C4 so an butanoate => D.

Qu35: E
Potassium salt => C or D, formyl is higher priority than Cl so gets the lower number giving E.

Qu36: C
Z double bond means B or C, but the ketone as 5-oxo rather than 4- implies C.

Qu37: D
There are only two chloro, nitro, acid chlorides which are D and E. The numbering is from the acid chloride. E is the 2,4-isomer.

Qu38: C
Both B and C have 2E,6Z double bonds. C is the R configuration and B is S.

Qu39: B
Only A and B are [4.4] spiro systems, the numbers start from next to the spiro C (which is the C that is common to both rings).

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