ORGANIC COMPOUNDS IN THREE DIMENSIONS

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Adrian Daniel
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1 (adapted from Blackburn et al., Laboratory Manual to Accompany World of hemistry, 2 nd ed., (1996) Saunders ollege Publishing: Fort Worth) Purpose: To become familiar with organic molecules in three dimensions and to understand isomerism of simple organic molecules. Introduction: Over 11 million of the more than 13 million known compounds are carbon compounds, and a separate branch of chemistry, organic chemistry, is devoted to the study of them. Two important reasons why there are so many organic compounds are (1) the ability of thousands of carbon atoms to be linked in sequence with stable carbon carbon bonds in a single molecule and (2) the occurrence of isomers. The basic geometrics for organic compounds are illustrated below: Tetrahedral Trigonal Planar Linear Aromatic There are four classes of hydrocarbons: alkanes, which contain single bonds with a tetrahedral arrangement around each carbon; alkenes, which contain one or more = double bonds with trigonal planar geometry around the carbon atoms with double bonds; alkynes, which contain one or more triple bonds with linear geometry around the carbon atoms with triple bonds; and the aromatics, which consist of benzene, benzene derivatives, and fused benzene rings. Revision S10 Page 1 of 11

Introduction: Over 11 million of the more than 13 million known compounds are carbon compounds, and a separate branch of chemistry, organic chemistry, is devoted to the study of them.

2 Structural Isomers: Two or more compounds with the same molecular formula but different arrangements of atoms are called isomers. Isomers differ in one or more physical or chemical properties such as boiling point, color, solubility, reactivity, and density. Several different types of isomerism are possible for organic compounds. These include structural isomers (straight chain and branched chain), cis and transisomers, and optical isomers. The simplest example of structural isomers is for 4 10 which can be either butane or methylpropane. butane ( 4 10 ) methylpropane ( 4 10 ) In the alkene series, the possibility of locating the double bond between two different carbon atoms adds additional structural isomers. Ethene and propene have only one possible location of the double bond. owever, the next alkene in the series, butene, has two possible locations for the double bond. 1 butene 2 butene Revision S10 Page 2 of 11

Several different types of isomerism are possible for organic compounds. These include structural isomers (straight chain and branched chain), cis and transisomers, and optical isomers.

3 is and Trans Isomers: An important difference between alkanes and alkenes is the degree of flexibility of the carbon carbon bonds in the molecules. Rotation around single carbon carbon bonds in alkanes occurs readily at room temperature, but the carbon carbon double bond in alkenes is strong enough to prevent free rotation about the bond. This leads to the possibility of cis trans isomers. is trans isomerism in alkenes is only possible when both of the double bond carbon atoms have two different groups. If two methyl groups replace two hydrogen atoms, one on each carbon atom of ethane ( 2 = 2 ), the result is 2 butene, 3 = 3. Experimental evidence confirms the existence of two compounds with the same set of bonds. The difference in the two compounds is in the location in space of the two methyl groups: the cis isomer has two methyl groups on the same side in the plane of the double bond and the trans isomer has two methyl groups on the opposite sides of the double bond. The two arrangements are distinguished from each other by the prefixes cis indicating groups on the same side and trans indicating groups on opposite sides. cis 2 butene trans 2 butene Optical Isomers: One class of optical isomers are found for compounds in which four different groups are attached to a given carbon atom as in XYZ. As can be seen from the images below, the resultant structures cannot be superimposed on each other no matter how they are turned about: X X Y Z Y Z mirror Revision S10 Page 3 of 11

This leads to the possibility of cis trans isomers. is trans isomerism in alkenes is only possible when both of the double bond carbon atoms have two different groups.

4 Equipment: molecular model kit Procedure: In this experiment you will make models of organic compounds using plastic balls to represent the atoms (with different colored balls for different kinds of atoms) and plastic rods to represent the chemical bonds. Using these as a guide, make the models called for and answer the questions. Also consult your textbook for additional information about the different kinds of organic isomers and the rules for naming organic compounds. Waste Disposal and leanup: Without breaking any of the parts, disassemble all parts of the molecular model kits and place them back in the container. Revision S10 Page 4 of 11

Using these as a guide, make the models called for and answer the questions.

5 DATA SEET Name: Partner: (1) Make a three dimensional model of ethane and draw its structure below. Are all six hydrogen atoms equivalent? Replace one hydrogen ball by a different colored plastic ball (representing Br). ow many isomers of ethyl bromide (bromoethane) are possible? Draw the structure(s) below. (2) Make a model of propane and draw its structure below. If you substitute one bromine atom for one hydrogen atom, how many isomers are possible? Draw their structures and name them. Revision S10 Page 5 of 11

ow many isomers of ethyl bromide (bromoethane) are possible? Draw the structure(s) below.

6 (3) ow many isomers are possible for an alkane with the formula 5 12? Draw all structures and name each one. (4) Make a model of 1,2 dibromoethene and draw its structure below. If cis trans isomers are possible, make models for them. Draw the structures below and name them. (5) Make a model of 2 methyl 1 butene. ow does this differ from 2 methyl 2 butene? Draw their structures. Revision S10 Page 6 of 11

(4) Make a model of 1,2 dibromoethene and draw its structure below.

7 (6) ow many butenes ( 4 8 ) are possible? Draw their structures and name them. (7) Make a model for the molecule lbrf. an the atoms be arranged in more than one way in three dimensions? If so, what is the relationship of the different arrangements? (What is the relationship of your face to the face you look at in the mirror every morning?) Draw structures of any arrangements that are different. (8) Make a model of 2 butene and draw its structure below. Are cis trans isomers possible? If so, draw the cis trans isomers and name each one. Revision S10 Page 7 of 11

(What is the relationship of your face to the face you look at in the mirror every morning?

8 (9) An alcohol is classified according to the kind of carbon atom which bears the O group. A carbon atom is classified as primary, secondary, or tertiary according to the number of alkyl groups attached to it. If the carbon atom which bears the O group has no or one alkyl group attached to it, the alcohol is a primary alcohol. If two alkyl groups are attached, the alcohol is a secondary alcohol; if three alkyl groups are attached, the alcohol is a tertiary alcohol. primary alcohol secondary alcohol tertiary alcohol In general, alcohols undergo reactions which are dependent on the type of alcohol used. For example, primary alcohols are oxidized more readily than are secondary alcohols, and tertiary alcohols are oxidized only under severe conditions. Other reactions show similar trends. Make models of primary, secondary, and tertiary alcohols that have the formula 4 9 O. Draw their structures below and name them. Revision S10 Page 8 of 11

If the carbon atom which bears the O group has no or one alkyl group attached to it, the alcohol is a primary alcohol.

9 POST LAB QUESTIONS Name: Partner: 1. Why does 2 butene have cis and trans isomers but 1 butene doesn t? 2. Draw the structural formula of 2 methyl 2 hexene. 3. Which of the following molecules can have optical isomers? (a) 2 l 2 (b) 2 N( 3 )OO (c) l(o) 2 l Revision S10 Page 9 of 11

3. Which of the following molecules can have optical isomers?

10 TIS PAGE INTENTIONALLY LEFT BLANK. Revision S10 Page 10 of 11

11 PRE LAB ASSIGNMENT Name: 1. Why is 4 10 the simplest alkane that can have structural isomers? 2. Draw the cis and trans isomers for l = l. Why are cis and trans isomers not possible for l 2 2 l? 3. Draw the optical isomers for lbrf. Why are the optical isomers not found for 2 lbr? Revision S10 Page 11 of 11

Molecular Models Experiment #1

Molecular Models Experiment #1 Objective: To become familiar with the 3-dimensional structure of organic molecules, especially the tetrahedral structure of alkyl carbon atoms and the planar structure of