MORISSON   Organic Chemistry

MORISSON Organic Chemistry


DisciplinaQuímica Orgânica I15.064 materiais278.309 seguidores
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measure various spectra of the compound
(Chap. 13), in particular the infrared spectrum and the nmr spectrum; indeed,
because of the wealth of information to be gotten in this way, spectroscopic examina-
tion might well be the first order of business after purification. From the mass
spectrum we would get a very accurate molecular weight.
We would carry out a qualitative elemental analysis to see what elements are
present (Sec. 2.25). We might follow this with a quantitative analysis, and from this
and the molecular weight we could calculate a molecular formula (Sec. 2.26); we
would certainly do this if the compound is suspected of being a new one.
Next, we study systematically the behavior of the compound toward certain
reagents. This behavior, taken with the elemental analysis, solubility properties,
and spectra, generally permits us to characterize the compound, that is, to decide
what family the unknown belongs to. We might find, for example, that the com-
pound is an alkane, or that it is an alkene, or an aldehyde, or an ester.
Now the question is: which alkane is it? Or which alkene, or which aldehyde,
or which ester? To find the answer, we first go to the chemical literature and look
up compounds of the particular family to which our unknown belongs.
If we find one described whose physical properties are identical with those of
our unknown, then the chances are good that the two compounds are identical.
For confirmation, we generally convert the unknown by a chemical reaction into
a new compound called a derivative, and show that this derivative is identical with
the product derived in the same way from the previously reported compound.
If, on the other hand, we do not find a compound described whose physical
112 ALKANES CHAP. 3
properties are identical with those of our unknown, then we have a difficult job
on our hands: we have a new compound, and must prove its structure. We may
carry out a degradation : break the molecule apart, identify the fragments, and deduce
what the structure must have been. To clinch any proof of structure, we attempt
to synthesize the unknown by a method that leaves no doubt about its structure.
Problem 3.22 The final step in the proof of structure of an unknown alkane
\\as its synthesis by the coupling of lithium di(ft>r/-butyl)copper with w-butyl bro-
mide. What \\as the alkane?
In Chap. 13, after we have become familiar with more features of organic
structure, we shall see how spectroscopy fits into the general procedure1 outlined
above.
3.33 Analysis of alkanes
An unknown compound is characterized as an alkane on the basis of negative
evidence.
Upon qualitative elemental analysis, an alkane gives negative tests for all
elements except carbon and hydrogen. A quantitative combustion, if one is carried
out, shows the absence of oxygen; taken with a molecular weight determination,
the combustion gives the molecular formula, -CnH 2n + 2 which is that of an alkane.
An alkane is insoluble not only in water but also in dilute acid and base and in
concentrated sulfuric acid. (As we shall see, most kinds of organic compounds
dissolve in one or more of these solvents.)
An alkane is unreactive toward most chemical reagents. Its infrared spectrum
lacks the absorption bands characteristic of groups ofatoms present in other families
of organic compounds (like OH, C~O, ,C C, etc.).
Once the unknown has been characterized as an alkane, there remains the
second half of the problem: finding out which alkane.
On the basis of its physical properties boiling point, melting point, density,
refractive index, and, most reliable of all, its infrared and mass spectra it may be
identified as a previously studied alkane of known structure.
If it turns out to be a new alkane, the proof of structure can be a difficult
job. Combustion and molecular weight determination give its molecular formula.
Clues about the arrangement of atoms are given by its infrared and nmr spectra.
(For compounds like alkanes, it may be necessary to lean heavily on x-ray diffrac-
tion and mass spectrometry.)
Final proof lies in synthesis of the unknown by a method that can lead only
to the particular structure assigned.
(The spectroscopic analysis of alkanes will be discussed in Sees. 13.15-13.16.)
PROBLEMS
1. Give the structural formula of:
(a) 2,2,3,3-tetramethylpentane (e) 2,4-dimethyl-4-ethylheptane
(b) 2,3-dimethyIbutane (f) 2,5-dimethylhexane
(c) 3,4,4,5-tetramethylheptane (g) 2-methyI-3-ethylpentane
(d) 3,4-dimethyI-4-ethylheptane (h) 2,2,4-trimethylpentane
PROBLEMS 113
2. Draw out the structural formula and give the IUPAC name of:
(a) (CH 3) 2CHCH2CH 2CH 3 (0 (CH 3)3CCH 2C(CH 3)3
(b) CH 3CH 2C(CH 3)2CH 2CH 3 (g) (CH 3)2CHCH 2CH 2CH(C2H 5)2
(c) (C2H 5)2C(CH 3)CH 2CH 3 (h) (CH 3)2CHCH(CH 3)CH 2C(C 2H 5)2CH 3
(d) CH 3CH 2CH(C H 3)CH(CH 3)CH(CH 3)2 (i) (CH 3 )2CHC(C 2H 5)2CH 2CH 2CH 3
CH 3 CH 3
(e) CH 3CH2CHCH 2CHCH 2CH 3 (j) CH 3CH 2CHCH 2CHCHCH 3
CH 3 CH 2CH 2CH 3 CH 2CH2CH 3
3. Pick out a compound in Problem 1 or 2 that has: (a) no tertiary hydrogen;
(b) one tertiary hydrogen; (c) two tertiary hydrogens; (d) no secondary hydrogen;
(e) two secondary hydrogens; (f) half the number of secondary hydrogens as primary
hydrogens.
4. Pick out a compound (if any) in Problem 1 or 2 that contains:
(a) one isopropyl group (g) one /*r/-butyl group
(b) two isopropyl groups (h) two tert-buiyl groups
(c) one isobutyl group (i) an isopropyl group and a j^r-butyl group
(d) two isobutyl groups (j) a /m-butyl group and an isobutyl group
(c) one sec-butyl group (k) a methyl, an ethyl, a //-propyl, and a jer-butyl
(f ) two .w-butyl groups group
5. What
alkane^
or alkancs of molecular weight 86 have: (a) two monobromo
derivatives? (b) three? (c) four? (d) five? (e) How many dibromo derivatives does the
alkane in (a) have?
6. How many mono-, di-, and trichloro derivatives are possible for cyclopentane ?
(Structure given in Sec. 9.5.)
7. Without referring to tables, list the following hydrocarbons in order of decreasing
boiling points (i e., highest boiling at top, lowest at bottom):
(a) 3.3-dimcth>lpcntane (c) 2-methylheptane (e) 2-methylhexane
(b) //-heptane (d) //-pentane
8. Wnte balanced equations, naming all organic products, for the following reac-
tions:
(a) isobutyl bromide f Mg/ether (d) product of (b) -f H 2O
(b) rm-butjl bromide + Mg/ether (e) product of (a) + D ;O
(c) product of (a) -i H :O (f) .wc-butyl chloride + Li, then Cul
(g) product of (f ) -f ethyl bromide
9. Write equations for the preparation of //-butane from:
(a) //-butyl bromide (d) l-butenc, CH,CH 2CH--CH 2
(b) .tec-butyl bromide (e) 2-butene, CH 3CH- CHCH 3
(c) ethyl chloride
10. Draw structures of all products expected from monochlorination at room tem-
perature of:
(a) if-hexane (c) 2,2,4-trimethylpentane
(b) isohexane (d) 2,2-dimethylbutane
11. Predict the proportions of products in the previous problem.
12. (a) Reaction of an aldehyde with a Grignard reagent is an important way of
making alcohols. Why must one scrupulously dry the aldehyde before adding it to the
Grignard reagent? (b) Why would one not prepare a Grignard reagent from
BrCH2CH 2OH?
114 ALKANES CHAP. 3
13. On the basis of bond strengths in Table 1.2, page 21, add the following free
radicals to the stability sequence of Sec. 3.24:
(a) vinyl, HiC-^CH-
(b) ally], H 2C=-CHCH 2 -
(c) benzyl, C6H 5CH 2 -
Check your answer on page 211.
14. On the basis of your answer to Problem 1 3, predict how the following would
fit into the sequence (Sec. 3.23) that shows ease of abstraction of hydrogen atoms:
(a) vinylic hydrogen, H 2O-CH--H
(b) allylic hydrogen, H 2C- CHCH 2-H
(c) benzylic hydrogen, C6H 5CH 2 H
Check your answer against the facts on page 210.
15. Free-radical chlorination of either w-propyl or isopropyl bromide gives 1-bromo-
2-chloropropane,