MORISSON   Organic Chemistry

MORISSON Organic Chemistry


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than the sum of their van der Waals radii; under these conditions, van der
Waals forces are repulsive (Sec. 1.19) and raise the energy of the conformation.
We say that there is van der Waals repulsion (or steric repulsion) between the
SEC. 3.6 HIGHER ALKANES. THE HOMOLOGOUS SERIES 79
Rotation >
Figure 3.4. Potential energy changes during rotation about C2 3 bond
of /7-butane.
methyl groups, and that the molecule is less stable because of van der Waals
strain (or steric strain).
Van der Waals strain can affect not only the relative stabilities of various
staggered conformations, but also the heights of the barriers between them. The
energy maximum reached when two methyl groups swing past each other rather
than past hydrogensis the highest rotational barrier of all, and has been estimated
at 4.4-6.1 kcal/mole. Even so, it is low enough that at ordinary temperatures, at
least the energy of molecular collisions causes rapid rotation; a given molecule
exists now in a gauche conformation, and the next instant in the ami conforma-
tion.
We shall return to the relationships among conformations like these of
/i-butane in Sec. 4.20.
Problem 3.3 Both calculations and experimental evidence indicate that the
dihedral angle between the methyl groups in the gauche conformation of /t-butane
is actually somewhat larger than 60. How would you account for this?
Problem 3.4 Considering only rotation about the bond shown, draw a potential
energy tu. rotation curve like Fig. 3.4 for: (a) (CH 3)2CH-CH(CH 3)2 ; (b) (CH3)2CH-
CH2CH 3 ; (c) (CH3)3C C(CH3)3 . (d) Compare the heights of the various energy
barriers with each other and with those in Fig. 3.4.
3.6 Higher alkanes. The homologous series
If we examine the molecular formulas of the alkanes we have so far considered,
we see that butane contains one caflki and two hydrogens more than propane,
80 ALKANES CHAP. 3
which in turn contains one carbon and two hydrogens more than ethane, and so on.
A series of compounds in which each member differs from the next member by a
constant amount is called a homologous series, and the members of the series are
called homologs. The family of alkanes forms such a homologous series, the
constant difference between successive members being CH 2 . We also notice that
in each of these alkanes the number of hydrogen atoms equals two more than
twice the number of carbon atoms, so that we may write as a generalformula for
members of this series, CnH2n + 2 . As we shall see later, other homologous series
have their own characteristic general formulas.
In agreement with this general formula, we find that the next alkane, pentane,
has the formula C5H 12 , followed by hexane, C6H 14 , heptane, C7H J6 , and so on.
We would expect that, as the number of atoms increases, so does the number of
possible arrangements of those atoms. As we go up the series of alkanes, we find
that this is true: the number of isomers of successive homologs increases at a
surprising rate. There are 3 isomeric pentanes, 5 hexanes, 9 heptanes, and 75
decancs (Ci ); for the twenty-carbon eicosane, there are 366,319 possible isomeric
structures! The carbon skeletons of the isomeric pentanes and hexanes are shown
below,
]
C
C-C-C-C-C c-C-C-C C-C-C Pentanes
tt-Pentane Isopentane Neopentane
b.p. 36 b.p. 28 b.p. 9.5
i j
C C
b.p. 69 b.p. 60 b.p. 63' 7/c/.tj
C
c-c-~c~c c c-c-c
! I I
C C C
b.p. 50 b p. 58
C
It is important to practice drawing the possible isomeric structures that corre-
spond to a single molecular formula. In doing this, a set of molecular models is
especially helpful since it will show that many structures which appear to be dif-
ferent when drawn on paper are actually identical.
Problem 3.5 Draw the structures of: (a) the nine isomeric heptanes (C7Hi$);
(b) the eight chloropentanes (C5 H,,C1); <c) the nine dibromobutanes (C4H8Br2).
3.7 Nomenclature
We have seen that the names methane, ethane, propane, butane, and pentane are
used for alkanes containing respectively one, two, three, four, and five carbon
atoms. Table 3.2 gives the names of madjjplarger alkanes. Except for the first
SEC. 3.8 ALKYL GROUPS 81
Table 3.2 NAMES OF ALKANES
CH4
C2H 6
C3H,
nonane
CioH 22 decane
CnH24 undecane
C] 2H 2o dodecane
CI^HSO let radecane
hexadecane
octadecane
methane
ethane
propane
butane
CsHi2 pentane
Coll 14 hcxanc
C?Hi6 heptane
CsH| 8 octane
four members of the family, the name is simply derived from the Greek (or Latin)
prefix for the particular number of carbons in the alkane; thus pentane for five,
hexane for six, heptane for seven, octane for eight, and so on.
The student should certainly memorize the names of at least the first ten
alkanes. Having done this, he has at the same time essentially learned the names
of the first ten alkenes, alkyncs, alcohols, etc., since the names of many families
of compounds are closely related. Compare, for example, the names propane,
propene, and propvne for the three-carbon alkane, alkene, and alkyne.
But nearly every alkane can have a number of isomeric structures, and there
must be an unambiguous name for each of these isomers. The butanes and pen-
tanes are distinguished by the use of prefixes: jj-butane and isobutane ; j-rjentane,
isopentanc, and neopentane^But there are 5 hexanes. 9Tieptanes, and 75 decanes;
it would be difficult to devise, and even more difficult to remember, a different
prefix for each of these isomers. It is obvious that some systematic method of
naming is needed.
As organic chemistry has developed, several different methods have been
devised to name the members of nearly every class of organic compounds; each
method was devised when the previously used system had been found inadequate
for the growing number of increasingly complex organic compounds. Unfortu-
nately for the student, perhaps, several systems have survived and are in current
use. Even if we are content ourselves to use only one system, we still have to under-
stand the names used by other chemists; hence it is necessary for us to learn more
than one system of nomenclature. But before we can do this, we must first learn
the names of certain organic groups.
3.8 Alkyl groups
In our study of inorganic chemistry, we found it useful to have names for
certain groups of atoms that compose only part of a molecule and yet appear
many times as a unit. For example, NH 4 * is called ammonium', NO 3 ~, nitrate;
SO 3
~
~, sulfite\ and so on.
In a similar way names are given to certain groups that constantly appear as
structural units of organic molecules. We have seen that chloromethane, CH 3C1,
is also known as methyl chloride. The CH* group is called methyl wherever it
appears, CH 3Br being methyl bromide, CH 3 I, methyi iodide, and CH 3OH, methyl
alcohol. In an analogous way, the C 2H 5 group is ethyl; C3H 7 , propyl; C4H9 ,
butyl; and so on.
These groups are named simply by dropping -ane from the name of the
corresponding alkane and replacing it by -yl. They are known collectively as
82 ALKANES CHAP. 3
alkyl groups. The general formula for an alkyl group is CnH2n +i , since it contains
one less hydrogen than the parent alkane, CnH 2n + 2 .
Among the alkyl groups we again encounter the problem of isomerism.
There is only one methyl chloride or ethyl chloride, and correspondingly only one
methyl group or ethyl group. We can see, however, that there are two propyl
chlorides. I and II, and hence that there must be two propyl groups. These groups
H H H - H H H
H C-C CCI H-C-C C-Hill lit
K H H H Cl H
I U
w-Propyl chloride Jsopropyl chloride
both contain the propane chain, but differ in the point of attachment of the
chlorine; they are called it-propyl and isopropyl: We can distinguish the two
CH 3CH 2CH 2 CH 3CHCH 3
w-Propyl
Isoprop> 1
chlorides by the names n-propyl chloride