Chem 401 Lab Exercise #5Nomenclature 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:

C

H

H

H

H C C

H

H

H

H

H

H

C C

H

H

H

H

H H

H

H

C

CH4 CH3CH3 CH3CH2CH3

or or

CH3 CH2 CH3CH3 CH3

Line Structureor Kekulé Structure

Condensed Structure

Skeletal Structure

1 C 2 C 3 C

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

atoms

Parent

Root

Name Condensed Structure

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:

CH3CH2CH2CH2CH3

a.

b.

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:

CH3CHCH2CH2CH2CHCH2CH2CH3

CH3

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

atoms

Parent Root Alkyl Group

Name

Condensed Structure

1 meth- methyl -CH3

2 eth- ethyl -CH2CH3

3 prop- propyl -CH2CH2CH3

4 but- butyl

5 pent- -CH2CH2CH2CH2CH3

6 hex-

4 cyclobut- cyclobutyl

6 cyclohex- cyclohexyl

10. Fill in the blanks in the above table.

CH3CHCH2CH2CH2CHCH2CH2CH3

CH3

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.

CH3CHCH2CH2CCH2CH2CH3

CH3

CH2CH3

CH3

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:

CH3CCH2CH2CH2CH3

CH2

CHCH2CH2CH2CH3

CH3

CH2CH3

Longest continuous chain is 11 C:so parent chain name is undecane.Note that the longest continuouschain is not necessarily in a straightline!

CH2CH2CH3

H3C

CH2CH2CH2CCH3

CH3

CH3

Cyclooctane is parent name; methyland propyl group are substituents

pentane is parent name as it has moreC atoms than cyclobutane;cyclobutyl group is substituent

12345

67 8 9 1 0 1 1

7CH3CH2CHCHCH2CH2CH3

CHCH3

CH3

CH3

1

2

3 4 5 6

Longest chain is 7 or heptane; there are 2substituents; a branched ethyl group at position 3 and a methyl group at position 4.

Longest chain is 7 or heptane; there are 3 substituents; anethyl group at position 3 and methyl groups at position 2 and 4. This would be the correct way to number this chain.

1 2 3 4 5 6CH3CH2CHCHCH2CH2CH3

CHCH3

CH3

CH37

2. Number the atoms in the parent chain so that the branches or alkyl groups have the lowest numbers possible.

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 forthe two methyl groups will be 2,4-dimethyl

7CH3CH2CHCHCH2CH2CH3

CHCH3

CH3

CH3

1

2

3 4 5 6

When there are multiple alkyl groups, check to make sure that you have assigned the lowest possible numbers.

7CH3CH2CHCHCH2CH2CH3

CHCH3

CH3

CH3

1

2

3 4 5 6

For this numbering system, the numbers forthe substituents are 2, 3, and 4. This would be the correct way to number this chain.

12345

6

CH3CH2CHCHCH2CH2CH3

CHCH3

CH3

CH3

7

For this numbering system, the numbers forthe substituents are 4, 5, and 6. These are notthe 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.

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)

The prefixes and names will be 3-ethyl and 2,4-dimethyl. In this case, the di does not count towardsalphabetizing as it is not part of the group name (group name is methyl, there are just 2 of them); soethyl comes before methyl. The complete name is: 3-ethyl-2,4-dimethylheptane

7CH3CH2CHCHCH2CH2CH3

CHCH3

CH3

CH3

1

2

3 4 5 6

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.

CH3CCH2CH2CH2CH3

CH2

CHCH2CH2CH2CH3

CH3

CH2CH3

CH2CH2CH3

H3C

CH2CH2CH2CCH3

CH3

CH3

24.

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.

CH3CH2CH2CH3 CH3CHCH3

CH3

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

Two groups are transCH3

CH3

Two groups are cis

Dashed arrow goes into paper

Solid arrow comes out of paper

CH3

CH3

CH3CH3

Two groups are trans as one points up and theother points down. Name is trans-1,2-dimethylcyclopentane.

Two groups are cis as both point in same direction. Name is cis-1,2-dimethylcyclopentane. Noticecis is in italicized in name.

CH3

No cis-trans isomerism asonly 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.

CH2CH3

CH3

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 cis-

trans isomerism, each C atom involved in the double bond must have 2 different groups attached to it (H included).

C

C

H

CH

3

H

H

3

C

C

C

CH

3

H

H

H

3

C

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.

Here the methyl groups are on opposite sides of

the double bond, so they are trans.

Name is

trans

-2-butene.

C

C

CH

2

CH

3

H

H

3

C

H

3

C

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.

Cl

CH3

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 propynepropene

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.

1-propanol dimethyl etherethyl methyl ether

OOOH

OH

2-propanol

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

propanal 2-propanone or just propanone

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

propanoic acid

OH

O

methyl propanoate

OCH3

O

propanamide

NH2

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