chapter 3 structure and stereochemistry of alkanes chapter 3: structure and stereochemistry of...

Chapter 3

STRUCTURE AND STEREOCHEMISTRY OF ALKANES

Chapter 3: Structure and Stereochemistry of Alkanes

IUPAC RULES

1. Form the base for the compounds’ name. This is the name of the longest continuous carbon chain (main chain) in the molecule. Other groups attached to the main chain are considered as substituents. If two chains of equal length can be found in a particular molecule, use the one that gives a greater number of substituents.

2. Number the main chain, starting with the end, which is nearest a substituent.


CH 3

CH 2

HC

CHHC

HC

CH 3

H3C

H3C CH 2

CH 3

CH 3

CH 3

CH 2

HC

CHHC

HC

CH 3

H3C

H3C CH 2

CH 3

CH 3

7-atom main chain (yellow color), four substituents - correct!!

7-atom main chain (yellow color), three substituents - incorrect!!

CH 3

CH 2

HC

CHHC

HC

CH 3

H3C

H3C CH 2

CH 3

CH 3

1

2

3

4 5

6

7

CH 3

CH 2

HC

CHHC

HC

CH 3

H3C

H3C CH 2

CH 3

CH 3

3-ethyl-2,4,5-trimethylheptanecorrect!!

7

6

5

4 3

2

1

5-ethyl-3,4,6-trimethylheptaneincorrect!!

RULE 1

RULE 2


IUPAC RULES (Contd.)

3. The substituents names are derived in the same fashion as the parent hydrocarbons, but instead of suffix –ane, put a suffix –yl. Assign numbers to the substituents matching their positions on the main chain.

The substituents also take prefixes, whenever necessary, to differentiate between different isomeric forms.

a) n-substituents; b) iso-substituents;

Both a) and b) have primary (1o) carbon atoms attached to the main chain.

c) sec-substituents;The carbon atom attached to the main chain is secondary (2o).

d) tert-substituents; The carbon atom attached to the main chain is tertiary (3o).


IUPAC RULES (Contd.)

4. Write the name as a single word, using hyphens to separate the different prefixes and using commas to separate the numbers. When two or more substituents are present, list the substituents in alphabetical order. If some of them are identical, list them together with a prefix identifying their total number: di- (two), tri- (three), tetra- (four), penta- (five), etc.

5. When a more complex substituent is encountered, one may need, for the naming of the substituent, to apply the entire procedure outlined above, i.e. select a main chain, etc. But as usual, the substituent as whole should always bear the suffix –yl, instead of –ane.


HEATS OF COMBUSTION AND RING STRAIN

RING SIZE

CYCLO

ALKANE

MOLAR HEAT

OF COMBUSTION

PER

CH2

GROUP

RING

STRAIN PER CH2

TOTAL RING STRAIN

3 Cyclopropane 499.8 kcal/mol 166.69.2 kcal/mol

27.6

4 Cyclobutane 655.9 164.0 6.6 26.4

5 Cyclopentane 793.5 158.7 1.3 6.5

6 Cyclohexane 944.5 157.4 0.0 0.0

7 Cycloheptane 1108.3 158.3 0.9 6.3

Ref.Long-chain alkane 157.4 0.0 0.0


STRAIN ENERGY VS. RING SIZE

0

5

10

15

20

25

30

3 4 5 6 7 8 9 10 11 12 13 14

Ring s ize

Str

ain

en

erg

y [

kc

al/

mo

l]


TYPES OF STRAIN IN ORGANIC MOLECULES (SUMMARY)

1. Torsional Strain: The strain due to eclipsing of bonds at neighboring carbon atoms. Energy cost – about 1 kcal/mol for C-H --- C-H eclipse, or 1.3 kcal/mol for C-H --- C-CH3 eclipse.

2. Steric Strain: The strain due to repulsive interactions, when atoms or groups approach each other too closely. Energy cost – about 0.9 kcal/mol for C-CH3 --- C-CH3 gauche interaction.

NOTE: The C-CH3 --- C-CH3 eclipsed interaction is a cumulative torsional + steric strain!!!

3. Angle Strain: The strain due to expansion or compression of bond angles. Energy cost – varies, depending on the actual bond angle.


chapter 3 structure and stereochemistry of alkanes chapter 3: structure and stereochemistry of...

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