Project  #25 Isomer Number Problem (Chapter 12)


Isomers of Alkanes, Alkyl halides, Alkenes, and Cycloalkanes   10 points


After reading chapter 12, isomerism is not discussed. This project is to look at structural isomerism. There is also geometric isomers (which is discussed) and optical isomerism in the study of organic chemistry.


To understand organic molecules you should have a grasp of molecular structure introduced in Chapter 6. Building structural isomers will strengthen your knowledge of building molecules. Usually it is helpful to have a molecular models kit, but you can assemble molecular structures with tooth picks and gum drops.


What is Structural Isomerism?

Structural isomerism, (or constitutional isomerism) is a form of isomerism in which molecules with the same molecular formula have bonded together in different orders making different compounds  Three categories of structural isomers are skeletal, positional, and functional isomers. Positional isomers are also called regioisomer.

In chain isomerism, or skeletal isomerism, components of the (usually carbon) skeleton are distinctly re-ordered to create different structures.









View the following links for additional help:

Alkane Prefixes: Count to Ten in Organic
Alkane Series
Alkyl Groups C1-C8
All Possible Alkyl Groups C1-C4



IUPAC Naming System for Organic Compounds




The following are additional web pages demonstrating the process to draw different isomers:


Worked Example: Naming Alkanes
Naming Alkanes-Page 2
Naming Alkanes-Page 3
Naming Alkanes-Page 4
Naming Alkanes-Page 5

Worked Example: Naming Alkenes
Naming Alkenes-Page 2
Naming Alkanes-Page 3

Isomer Number Problems
Three Pentane Isomers
Writing Isomers 1
Writing Isomers 2




Another Worked Example:



Look at the following two examples:





Structural Isomers: Alkanes:


IUPAC Rules for Alkane Nomenclature

 1.   Find and name the longest continuous carbon chain.
 2.   Identify and name groups attached to this chain.
 3.   Number the chain consecutively, starting at the end nearest a substituent group.
 4.   Designate the location of each substituent group by an appropriate number and name.
 5.   Assemble the name, listing groups in alphabetical order using the full name (e.g. cyclopropyl before isobutyl).
    The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when alphabetizing.


Sample Video for drawing Structural isomers of alkanes:



Structural Isomers: Alkyl halides:
The halogen is treated as a substituent on an alkane chain. The halo- substituent is considered of equal rank with an alkyl substituent in the numbering of the parent chain. The halogens are represented as follows:









Here are some examples:


  Worked Example:  C4H9Br  4 isomers (Bromobutanes);




Worked Example:  C3H6Br2  4 isomers (Dibromopropanes); 









Structural Isomers: Cycloalkanes

      Cycloalkanes have one or more rings of carbon atoms. The simplest examples of this class consist of a single, unsubstituted carbon ring, and these form a homologous series similar to the unbranched alkanes. The IUPAC names of the first five members of this series are given in the following table. The last (yellow shaded) column gives the general formula for a cycloalkane of any size. If a simple unbranched alkane is converted to a cycloalkane two hydrogen atoms, one from each end of the chain, must be lost. Hence the general formula for a cycloalkane composed of n carbons is CnH2n. Although a cycloalkane has two fewer hydrogens than the equivalent alkane, each carbon is bonded to four other atoms so such compounds are still considered to be saturated with hydrogen.

Examples of Simple Cycloalkanes


















Substituted cycloalkanes are named in a fashion very similar to that used for naming branched alkanes. The chief difference in the rules and procedures occurs in the numbering system. Since all the carbons of a ring are equivalent (a ring has no ends like a chain does), the numbering starts at a substituted ring atom.

IUPAC Rules for Cycloalkane Nomenclature

 1.   For a monosubstituted cycloalkane the ring supplies the root name (table above) and the substituent group is named as usual. A location number is unnecessary.
 2.   If the alkyl substituent is large and/or complex, the ring may be named as a substituent group on an alkane.
 3.   If two different substituents are present on the ring, they are listed in alphabetical order, and the first cited substituent is assigned to carbon #1. The numbering of ring carbons then continues in a direction (clockwise or counter-clockwise) that affords the second substituent the lower possible location number.
 4.   If several substituents are present on the ring, they are listed in alphabetical order. Location numbers are assigned to the substituents so that one of them is at carbon #1 and the other locations have the lowest possible numbers, counting in either a clockwise or counter-clockwise direction.
 5.   The name is assembled, listing groups in alphabetical order and giving each group (if there are two or more) a location number. The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when alphabetizing.


Worked Examples: C5H10  5 isomers (cycloalkanes only)




Structural Isomers:  Alkenes

IUPAC Rules for Alkene and Cycloalkene Nomenclature

 1.   The ene suffix (ending) indicates an alkene or cycloalkene.
 2.   The longest chain chosen for the root name must include both carbon atoms of the double bond.
 3.   The root chain must be numbered
from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
 4.   The smaller of the two numbers designating the carbon atoms of the double bond is used as the double bond locator. If more than one double bond is present the compound is named as a diene, triene or equivalent prefix indicating the number of double bonds, and each double bond is assigned a locator number.
 5.   In cycloalkenes the double bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.
 6.   Substituent groups containing double bonds are:
2C=CH–   Vinyl group
2C=CH–CH2   Allyl group

Worked Examples: C5H10  5 isomers (alkenes only)


                          (There is no 3-Pentene)







Project #25 Assignment:


Draw the structural or semi-structural formulas for all the isomers of the following chemical formulas listed below, then give the IUPAC name for each:


You may do it on separate paper or fill out the following Isomer # Report Form:

Submit hard copy by the last class meeting, or submit electronic copy the Last day of the term.


#1  Alkane: C6H14   5 isomers - 1 point

   Watch Video:



#2   Alkane  C7H16        9 isomers - 2 points        


#3   Alkyl Halide: C5H11Br     8 isomers - 2 points  (See Example C4H9Br above)


#4  Alky Halide (Dienes)  C3H6Br2   9 isomers - 2 points (See Example C4H9Br above)


#5  Cycloalkanes C6H12  12 isomers (cycloalkanes only) - 2 points (See Example C5H10 above)


#6 C6H12  Alkenes 13 isomers (alkenes only) - 2 points (See Example C5H10 above)



#7 C4H10O Alcohols and Ethers  7 isomers (Alcohols & Ethers) – 1 point


(Answer Links made available after Homework is submitted)