MORISSON   Organic Chemistry

MORISSON Organic Chemistry


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for:
(a) (d) H2 + X 2 > 2HX, where X = F, Cl, Br, I
(e) C2H 6 + Br2 > C2H,Br + HBr
(f) C6H 5CH 3 -f Br2 > C\H 5CH2Br + HBr
(g) H2C -CHCH 3 + Br2 > H 2O=CHCH 2Br + HBr
(h) Reactions (e), (f), and (g) proceed by the same free radical mechanism as halo-
genation of methane. Calculate A// for each step in these three reactions.
12. A conceivable mechanism for the chlorination of methane involves the following
steps :
(1) C1 2 v 2C1-
(2) Cl- + CH4 > CH 3C1 + H.
(3) H- + C1 2 > HC1 + Cl-
then (2), (3), (2), (3), etc.
(a) Calculate Af/ for each of these steps, (b) Why does this mechanism seem less likely
than the accepted one given in Sec. 2.12? (Additional, conclusive evidence against this
alternative mechanism will be presented in Sec. 7,10.)
13. (a) Free methyl radicals react with methane as follows:
(/) CH 3 - + CH4 > CH 4 + CHr
On the basis of the bond strengths involved, show why the above reaction takes place
rather than the following:
(//) CHr + CH4 > CH3-CH 3 + H-
(b) Reaction (/) has an Eact of 13 kcal. In Sec. 2.12 it was listed as probable (but un-
productive) on grounds of collision probability. In actuality, how probable is reaction
(/) in, say, a 50:50 mixture of CH4 and C1 2 ? (Hint: See Sees. 2.20 and 2.18.)
14. Bromination of methane is slowed down by addition of fairly large amounts of
HBr. (a) Suggest a possible explanation for this. (Hint: See Sec. 2.17.) (b) Account for
the fact that HC1 does not have a similar effect upon chlorination. (c) Any reaction tends
to slow down as reactants are used up and their concentrations decrease. How do you
account for the fact that bromination of methane slows down to an unusually great
extent, more than, say, chlorination of methane?
15. A mixture of H2 and C1 2 does not react in the dark at room temperature. At
high temperatures or under the influence of light (of a wavelength absorbed by chlorine)
a violent reaction occurs and HC1 is formed. The photochemical reaction yields as
many as a million molecules of HC1 for each photon absorbed. The presence of a small
amount of oxygen slows down the reaction markedly, (a) Outline a possible mechanism
to account for these facts, (b) Account for the fact that a mixture of H2 and I2 does not
72 METHANE CHAP. 2
behave in the same way. (Hydrogen iodide is actually formed, but by an entirely different
mechanism.)
16. A stream of tetramethyllead vapor, (CH3)4Pb, was passed through a quartz
tube which was heated at one spot; a mirror of metallic lead was deposited at the hot
point, and the gas escaping from the tube was found to be chiefly ethane. The tube was
next heated upstream of the lead mirror while more tetramethyllead was passed through;
a new mirror appeared at the hot point, the old mirror disappeared, and the gas escaping
from the tube was now found to be chiefly tetramethyllead. Experiments like this, done
by Fritz Paneth at the University of Berlin, were considered the first good evidence for
the existence of short-lived free radicals like methyl, (a) Show how these experimental
results can be accounted for in terms of intermediate free radicals, (b) The farther up-
stream the tube was heated, the more slowly the old mirror disappeared. Account for this.
17. When a small amount (0.02%) of tetraethyllead, (C2H5)4Pb, is added to a mix-
ture of methane and chlorine, chlorination takes place at only 140 instead of the usual
minimum of 250. In light of Problem 16, show how this fact strengthens the mechanism
of Sec. 2.12.
Chapter Alkanes
Free- Radical Substitution
3.1 Classification by structure: the family
The basis of organic chemistry, we have said, is the structural theory. We
separate all organic compounds into a number of families on the basis of structure.
Having done this, ,we find that we have at the same time classified the compounds
as to their physical and chemical properties. A particular set of properties is thus
characteristic of a particular kind of structure.
Within a family there are variations in properties. All members of the family
may, for example, react with a particular reagent, but some may react more readily
than others. Within a single cdmpound there may be variations in properties,
one part of a molecule being more reactive than another part. These variations
in properties correspond to variations in structure.
As we take up each family of organic compounds, we shall first see what
structure and properties are characteristic of the family. Next we shall see how
structure and properties vary within the family. We shall not simply memorize
these facts, but. whenever possible, shall try to understand properties in terms of
structure, and to understand variations in properties in terms of variations in
structure.
Having studied methane in s.ome detail, let us now look at the more compli-
cated members of the alkane family. These hydrocarbons have been assigned
to the same family as methane on the basis of their structure, and on the whole
their properties follow the pattern laid down by methane. However, certain new
points will arise simply because of the greater size and complexity of these com-
pounds.
3.2 Structure of ethane
Next in size after methane is ethane, C2H6 . If we connect the atoms of this,
molecule by covalent bonds, following the rule of one bond (one pair of electron?*
73
74 ALKANES CHAP. 3
for each hydrogen and four bonds (four pairs of electrons) for each carbon, we
arrive at the structure
HH
^ *f
H:C:C:H H C C H
HH
^ h
Ethane
Each carbon is bonded to three hydrogens and to the other carbon.
Since each carbon atom is bonded to four other atoms, its bonding orbitals
(sp* orbitals) are directed toward the corners of a tetrahedron. As in the case of
methane, the carbon-hydrogen bonds result from overlap of these*^3 orbitals
with the s orbitals of the hydrogens. The carbon-carbon bond arises from over-
lap of two sp* orbitals.
The carbon-hydrogen and carbon-carbon bonds have the same general
electron distribution, being cylindrically symmetrical about a line joining the
atomic nuclei (see Fig. 3.1); because of this similarity in shape, the bonds are given
the same name, a bonds (sigma bonds).
Figure 3.1. Ethane molecule. Carbon-
carbon single bond: a bond.
Figure 3.2. Ethane molecule : shape and
size.
In ethane, then, the bond angles and carbon-hydrogen bond lengths should
be very much the same as in methane, that is, about 109.5 and about 1.10 A,
respectively. Electron diffraction and spectroscopic studies have verified this
structure in all respects, giving (Fig. 3.2) the following measurements for the
molecule: bond angles, 109.5; C H length, 1.10 A; C C length, 1.53 A. Simi-
lar studies have shown that, with only slight variations, these values are quite
characteristic of carbon-hydrogen and carbon-carbon bonds and of carbon bond
angles in alkanes.
3.3 Free rotation about the carbon-carbon single bond. Conformations.
Torsional strain
This particular set of bond angles and bond lengths still does not limit us to a
single arrangement of atoms for the ethane molecule, since the relationship
between the hydrogens of one carbon and the hydrogens of the other carbon is
not specified. We could have an arrangement like I in which the hydrogens exactly
oppose each other, an arrangement like II in which the hydrogens are perfectly
staggered, or an infinity of intermediate arrangements. Which of these is the
actual structure of ethane ? The answer is : all of them.
We have seen that the a bond joining the carbon atoms is cylindrically sym-
netrical about a line joining the two carbon nuclei; overlap and hence bond
SEC. 3.3 FREE ROTATION ABOUT THE CARBON-CARBON SINGLE BOND 75
I
Eclipsed conformation
H ii
Staggered conformation
Ethane
strength should be the same for all these possible arrangements. If the various
arrangements