Download Chem 401 Lab Exercise #5 Nomenclature Worksheet for Alkanes

Chem 401 Lab Exercise #5
Nomenclature Worksheet for Alkanes and Cycloalkanes
Structure and Nomenclature of Alkanes
Alkanes are saturated hydrocarbons; that is, they contain only C and H which are bonded solely with σ bonds. As
the valence of C is 4 and the valence of H is 1, C will have 4 bonds and H may only have 1. Because following this
rule (C will get 8 electrons or 4 bonds; H will get 2 electrons or 1 bond), we don’t bother counting valence electrons.
Alkanes have the general formula CnH2n+2.
The simplest alkanes are continuous chain alkanes. They are also called straight chain alkanes, although the chain
is not really straight, it zig-zags! In continuous chain alkanes, the C atoms are connected together in a line, and the H
atoms fill around. Here are the structural formulas for the 3 simplest continuous chain alkanes:
Line Structure
or Kekulé Structure
H
1C
2C
H
H H
C H
H
Condensed Structure
CH4
H H
H
C C
C H
H H
H H
H
CH3CH3
or
CH 3CH2CH3
or
H C C H
CH3 CH 3
Skeletal Structure
3C
H
CH 3 CH2 CH3
N/A
Note that there is more than one way to draw or write the structure. The Kekulé structure would be like the Lewis
structure. The condensed structure doesn’t show all the bonds: the line bonds between C and H are omitted; and
even the line bonds between C atoms may be dropped. Instead, due to the predictability of C and H bonds, the C-C
and C-H bonds are understood. Notice that in the condensed structure, the H atoms immediately follow the C to
which they are attached. In the skeletal structure, the H atoms are omitted altogether, and the C atoms are implied
wherever a line ends or at the intersection of 2 lines. The reader can easily determine how many H atoms are
attached to each C as the total number of bonds must equal 4.
1. Draw the Kekulé, condensed, and skeletal structure for the continuous chain alkane with the formula C6H14.
2. What is the molecular formula for the following skeletal structure?
In naming straight chain alkanes, we combine a parent root with the suffix -ane. The parent root and names for the
most common alkanes are below. You should memorize these parent root and names. The parent root for 5 C and
above come from the Greek or Latin names for numbers, while the parent root for 1 to 4 C are common names.
Table: Parent Names of Continuous (Straight) Chain Alkanes
# of C
Parent
Name
Condensed Structure
atoms
Root
1
meth-
methane
CH4
2
eth-
ethane
CH3CH3
3
prop-
propane
CH3CH2CH3
4
but-
butane
CH3CH2CH2CH3
5
pent-
pentane
CH3CH2CH2CH2CH3
6
hex-
hexane
CH3CH2CH2CH2CH2CH3
7
hept-
heptane
CH3CH2CH2CH2CH2CH2CH3
8
oct-
octane
CH3CH2CH2CH2CH2CH2CH2CH3
9
non-
nonane
CH3CH2CH2CH2CH2CH2CH2CH2CH3
10
dec-
decane
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
11
undec-
undecane
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3
12
dodec-
dodecane
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3
20
ico-
icosane
CH3(CH2)18CH3
3. Write the Kekulé structure for decane.
4. Name the following alkanes:
a.
b.
CH3CH2CH2CH2CH3
Cycloalkanes
Cyclic alkanes are hydrocarbon chains where two end C atoms join to form a ring. Because of this extra C-C bond,
two H atoms are lost in simple cycloalkanes. So the general formula is CnH2n. To name simple cycloalkanes, we add
the prefix cyclo to the parent name for the corresponding continuous chain alkane. So cyclopropane is:
5. Draw the structure for cyclohexane.
Alkyl Groups and Branched Alkanes
There are many alkanes which do not have a simple straight chain structure. Instead, they are what we call branched
alkanes. In branched alkanes, 1 or more H atoms are substituted with alkyl groups or alkyl substituents. Branched
alkanes are named after the parent name for the longest continuous chain alkane present in the molecule. For
example, consider the following structure:
CH3
CH3CHCH2CH2CH2CHCH2CH2CH3
CH2CH2CH2CH3
6. Draw a line through the longest continuous chain of C atoms.
7. How many C atoms are in the longest continuous chain (main chain)?________
8. What is the parent name for the main chain? _______________
9. How many branches or alkyl groups are there on the main chain?________
How do you name a more complex branched alkane when it has alkyl (or cycloalkyl) groups? The alkyl group
substituents are named by taking the parent root and and adding the suffix -yl. All chemists recognize that the -yl
ending means that this is an alkyl substituent. See the following Table to see the trend. Once the alkyl groups are
identified, they are added as prefixes to the parent alkane name. You will learn how in the following section.
Table: A Few Common Alkyl Groups
# of C
Parent Root
atoms
Alkyl Group
Condensed Structure
Name
1
meth-
methyl
-CH3
2
eth-
ethyl
-CH2CH3
3
prop-
propyl
-CH2CH2CH3
4
but-
butyl
5
pent-
6
hex-
4
cyclobut-
cyclobutyl
6
cyclohex-
cyclohexyl
-CH2CH2CH2CH2CH3
10. Fill in the blanks in the above table.
CH3
CH3CHCH2CH2CH2CHCH2CH2CH3
CH2CH2CH2CH3
11. In the above structure (used in 6-10), what are the names of the 2 alkyl groups?__________ and __________.
Use the following structure for Q 12-15.
CH3
CH3
CH3CHCH2CH2CCH2CH2CH3
CH2CH3
12. How many C atoms are in the longest continuous chain (main chain)?________
13. What is the parent name for the main chain? _______________
14. How many branches or alkyl groups are there on the main chain?________
15. The names for the alkyl groups are ____________________________________________.
Rules for Naming Branched Alkanes (and Cyclalkanes)
1. Find the longest continuous chain. This is the main or parent chain. If there is a ring with more C atoms than
the longest continuous chain, it is considered the parent chain. If the ring contains fewer C than the longest
continuous chain, then the ring is treated as a cycloalkyl group. If 2 or more chains of equal length may be
identified, the parent chain should be the chain containing more branches.
Examples:
CH2CH2 CH3
CH3
4 3
2
1
5 CHCH2CH2 CH 2CH3
6 CH 2
9
7 8
10 11
CH3 CCH2CH2 CH2CH3
CH2CH3
H3C
Cyclooctane is parent name; methyl
and propyl group are substituents
CH3
CH2 CH2 CH2CCH 3
Longest continuous chain is 11 C:
so parent chain name is undecane.
Note that the longest continuous
chain is not necessarily in a straight
line!
CH 3
CHCH 3
CH 3 CH 2 CHCHCH 2 CH 2 CH 3
6
1
2
3 4
5
7
CH 3
Longest chain is 7 or heptane; there are 2
substituents; a branched ethyl group at position
3 and a methyl group at position 4.
CH3
pentane is parent name as it has more
C atoms than cyclobutane;
cyclobutyl group is substituent
1 CH 3
2 CHCH 3
CH 3 CH 2 CHCHCH 2 CH 2 CH 3
5
3 4
6
7
CH 3
Longest chain is 7 or heptane; there are 3 substituents; an
ethyl group at position 3 and methyl groups at position 2
and 4. This would be the correct way to number this chain.
2. Number the atoms in the parent chain so that the branches or alkyl groups have the lowest numbers possible.
When there are multiple alkyl groups, check to make sure that you have assigned the lowest possible numbers.
1 CH 3
7CH 3
2 CHCH 3
6 CHCH 3
CH 3 CH 2 CHCHCH 2 CH 2 CH 3
5
3 4
6
7
CH 3
For this numbering system, the numbers for
the substituents are 2, 3, and 4. This would
be the correct way to number this chain.
CH 3 CH 2 CHCHCH 2 CH 2 CH 3
3
1
2
5 4
CH 3
For this numbering system, the numbers for
the substituents are 4, 5, and 6. These are not
the lowest numbers possible.
3. Number and name the substituents. Both the number and the name will be used in naming the compound. If
there are more than 1 of the same substituent, a prefix such as di, tri, tetra, etc. is used before the name of the
group. The numbers for the repeated groups would be separated by commas.
1 CH 3
2 CHCH 3
CH 3 CH 2 CHCHCH 2 CH 2 CH 3
5
3 4
6
7
CH 3
There are methyl groups at position 2 and 4, and there is an ethyl group at position 3.
The prefix and name for the ethyl group will be 3-ethyl; while the prefix and name for
the two methyl groups will be 2,4-dimethyl
4. Attach the number and name of the substituents to the name of the parent chain. Write the full compound
name as a single name: use hyphens to separate numbers from substituents (i.e. 3-ethyl-2-methyl), and numbers
are separated by commas (i.e. 2, 4-dimethyl). If there are multiple substituents, the substituent names are placed
in alphabetical order, regardless of their numbered position (so ethyl comes before methyl). For alphabetizing,
only the substituent name is considered, do not consider the multiplying prefixes di, tri, tetra, etc. These only tell
you how many of this substituent there are, they are not the parent name of the group. (so 5-ethyl-2,2-dimethyl
would be correct as ethyl comes before methyl; 2,2-dimethyl-5-ethyl would be incorrect)
1 CH 3
2 CHCH 3
CH 3 CH 2 CHCHCH 2 CH 2 CH 3
5
3 4
6
7
CH 3
The prefixes and names will be 3-ethyl and 2,4-dimethyl. In this case, the di does not count towards
alphabetizing as it is not part of the group name (group name is methyl, there are just 2 of them); so
ethyl comes before methyl. The complete name is:
3-ethyl-2,4-dimethylheptane
Use the following structure for Q 16-20.
16. How many C atoms are in the longest continuous chain (main chain)?________
17. What is the parent name for the main chain? _______________
18. The name of the substituent is ________ and its numbered position (give lowest possible number) is _____.
19. The number of the substituent is separated by a ________ from its name, while there is no separation between
the substituent name and the name of the parent chain.
20. The name for this compound is ____________________________________________.
21. Draw a structure for the following compounds:
a. 3-methylpentane
b. 1-propylcyclopentane
c. 4,5-diethyl-2-methyldodecane
Name the following compounds.
22.
23.
CH3
CHCH2CH2CH2CH3
CH2
CH3CCH2CH2CH2CH3
CH2CH3
24.
CH3
CH2CH2CH2CCH3
CH3
CH2CH2CH3
H 3C
Isomers: Structural (Constitutional) Isomers and cis, trans Stereoisomers
Two or more different compounds which have the same molecular formula are called isomers. Structural (also called
constitutional) isomers are compounds in which the atoms are bonded in a different order. Here are the two
structural isomers possible for the molecular formula C4H10.
CH3
CH3CH2CH2CH3
CH3CHCH3
25. Name the above isomers.
26. Draw and name all possible isomers of C5H12.
27. Draw and name all possible isomers of C5H10.
Another type of isomer are stereoisomers. Stereoisomers have the same molecular formula, and the atoms are
connected in the same order (same bond connectivity), BUT have different orientations in space. So the atoms are
arranged differently in space.
There are several types of stereoisomers, but the simplest type is called cis-trans stereoisomers (also called
geometric isomers). This type of stereoisomer occurs when there is a source of rigidity in the molecule.
Cycloalkanes and alkenes both exhibit cis-trans isomers. In cycloalkanes, the closed ring structure restricts rotation
about the C-C bond, so the 2 substituents on each ring C may point up or down. When we draw rings, we can show
this up vs. down by drawing the connecting lines so that it is clear that one group is pointing up while the other
group is pointing down. In order for cycloalkanes to show cis-trans isomerism, at least 2 ring C atoms must have 2
different groups attached. Some examples follow.
CH3
CH3
CH3
Two groups are trans
CH3
Two groups are cis
CH3
Dashed arrow goes into paper
Solid arrow comes out of paper
CH3
CH3
CH3
Two groups are cis as both point in same direction.
Name is cis-1,2-dimethylcyclopentane. Notice
cis is in italicized in name.
Two groups are trans as one points up and the
other points down. Name is
trans-1,2-dimethylcyclopentane.
CH3
No cis-trans isomerism as
only 1 C has 2 different groups.
28. Name the following compound (remember that when equal numbering systems are possible, give the lower
numbers in alphabetical order.
CH3
CH2CH3
29. Draw the correct structure for cis-1-methyl-4-propylcyclohexane.
Alkenes contain at least 1 double bond. The π bond in the double bond restricts C-C rotation, so the two substituents
on each of the double bond carbons are fixed in relation to each other. Substituents may be on the same side of the
double bond (cis) or they may be on opposites sides of the double bond (trans). In order for alkenes to show cistrans isomerism, each C atom involved in the double bond must have 2 different groups attached to it (H included).
H3C
C C
H
H3C
CH 3
H
2 different groups on each double bond C.
The methyl groups are on the same side of
the double bond, so they are cis.
Name is cis -2-butene. Note that the longest chain is
a but- chain with a double bond in the number 2
position. Therefore, this gives the 2-butene portion
of the name. So going from an alkane to an alkene
changes the suffix from -ane to -ene.
H
C C
CH 3
H
Here the methyl groups are on opposite sides of
the double bond, so they are trans.
Name is trans -2-butene.
H3C
H3C
H
C C
CH 2 CH 3
The first double bond C has 2 methyl groups, so no
isomerism possible. Name is 2-methyl-2-pentene.
30. Draw and label the cis and trans isomers of 3-hexene. (The double bond STARTS at the 3rd C.)
Alkyl Halides
Halogens are a common alkane substituent. It is quite easy to name alkyl halides as you treat the halide group(s)
exactly as you would treat an alkyl group. For example, in the following structure you treat the chloro group at
position 2 the same as any alkyl group. So the name is 2-chloro-3-methylhexane.
CH3
Cl
31. Draw the structure for 2-chloro-2,3-dimethylbutane.
32. Name the following compound.
Br
Other Functional Groups
Although there are many functional groups, there are several which are most important for you to know at this point.
They are: alcohols, ethers, carboxylic acids, ketones, aldehydes, esters, amides, alkenes, and alkynes.
Family Name
Group Name
Group Structure
General Formula
Name Ending
alkane
alkyl
C-C
R3*C-CR3*
-ane
alkene
double bond; alkenyl
C=C
R2*C=CR2*
-ene
alkyne
triple bond; alkynyl
C≡C
R*C≡CR*
-yne
alcohol
hydroxyl
-OH
ROH
-ol
ether
alkoxyl
-OR
ROR
ether
carboxylic acid
carboxy
-CO2 H
R*CO2 H
-oic acid
ester
alkoxycarbonyl
-CO2 R
R*CO2 R
-oate
ketone
oxo
-C(=O)R
RC(=O)R
-one
aldehyde
oxo
-C(=O)H
R*C(=O)H
-al
amide
amido
-C(=O)NH2
R*C(=O)NH2
-amide
* Here the R’s may be H atoms or alkyl groups. If no asterisk, then the R must be an alkyl group and can’t be H.
So when you go from an alkane to an alkene, the -ane ending becomes -ene. So the name ending basically tells you
what the most important functional group is. (Many compounds contain more than 1 functional group, so the
naming is more complicated. For now, you will only be expected to be able to name simple compounds.)
For example, here are the structures and names for some simple compounds:
propane
propene
propyne
Do you see how the name changes as you go from a single to a double to a triple bond? Also, do you see that the
alkyne drawn above correctly shows its linear geometry around the triple bond C’s? Remember, these triple bond C
atoms are sp hybridized, giving a linear geometry.
OH
OH
1-propanol
O
O
2-propanol
ethyl methyl ether
dimethyl ether
In the above 4 structures, the first 2 are alcohols while the other 2 are ethers. An -OH group is the alcohol group, so
the name becomes -ol. Also note, that for alcohols with a hydrocarbon chain of 3 or longer, the position of the -OH
group must be specified. The ethers are more complex (don’t worry about naming them now), but do you see the
C-O-C linkage? This is the hallmark of an ether group.
O
O
2-propanone or just propanone
propanal
Aldehydes get an -al ending and are very simple to recognize. They have the C=O group at the end of the alkyl
chain so there is always an H attached to the C=O carbon (this carbon is called the carbonyl carbon). In the above
structure for propanal, the end H is not shown, but if you count the bonds on the end C, you should realize that there
is 1 H attached to it. Ketones are also simple to recognize with the R2C=O group. Now neither R group can be an H
as that would make it an aldehyde. So the difference between ketones and aldehydes is that aldehydes have the
carbonyl C on the end of a chain, while ketones have the carbonyl C inside the chain. For ketones, this means that
for C chains of 5 or more carbons, the position of the C=O must be specified. For C chains of 3 or 4, it may be
specified as above, but it is not strictly necessary.
O
O
OH
propanoic acid
O
NH2
propanamide
OCH3
methyl propanoate
The carboxylic acids have a RC(=O)OH ending [the parenthesis here means that the =O is attached to the C right
before this (=O). The -OH group is also attached to the same C, the carbonyl C]. So the carbonyl C is on the end
like an aldehyde, but the H of an aldehyde becomes an -OH group. So an acid is on the end of a chain. Amides and
esters are derived from carboxylic acids and can get more complicated to name (so don’t worry now). Just be able
to recognize the group. For simple amides, the -OH group of a carboxylic acid becomes an -NH2 group (these H’s
can also be alkyl groups). For esters, the -OH group of a carboxylic acid becomes an -OR group like -OCH3 or
-OCH2CH3. This is basically an ether ending. This ether ending gets named first (for simple esters).
Draw the following compounds:
33. Ethanol
34. Ethanoic acid (acetic acid)
35. Butanal
36. 2-pentanone
37. 3-pentanone
38. 3-hexanol
39. 2-hexanol
40. Pentanal