drawing chemical structures condensed structures: c-h and c-c and single bonds aren't shown but...

Drawing Chemical Structures • Condensed structures: C-H and C-C and single bonds aren't shown but

understood

– If C has 3 H’s bonded to it, write CH3

– If C has 2 H’s bonded to it, write CH2; and so on. Sometimes bonds between carbons aren't shown in condensed structures—here the CH3, CH2, and CH units are simply drawn next to one another, but some bonds are added for clarity The compound called 2-methylbutane, for example, is written as follows:

Constitutional Isomers• Isomers that differ in how their atoms are arranged in chains are called

constitutional isomers• Compounds other than alkanes can be constitutional isomers of one

another• They must have the same molecular formula to be isomers

Alkanes and Alkane Isomers• Alkanes: Compounds with C-C single bonds and C-H bonds

only (no functional groups)• Connecting carbons can lead to large or small molecules• The formula for an alkane with no rings in it must be CnH2n+2

where the number of C’s is n• Alkanes are saturated with hydrogen (no more can be added• They are also called aliphatic compounds

Alkane Isomers

• CH4 = methane, C2H6 = ethane, C3H8= propane

• The molecular formula of an alkane with more than three carbons can give more than one structure– C4 (butane) = butane and isobutane

– C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane

• Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes

• Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes

Names of Small HydrocarbonsNo. of Carbons

Formula Name (CnH2n+2)

1 Methane CH4

2 Ethane C2H6

3 Propane C3H8

4 Butane C4H10

5 Pentane C5H12

6 Hexane C6H14

7 Heptane C7H16

8 Octane C8H18

9 Nonane C9H20

10 Decane C10H22

Alkane Structures

Planar zig-zag

condensed-line condensed

A perfect 3-D zig-zag forn-butane

Isomers: butane (C4H10)

Isomers: pentane (C5H12)

Which are isomeric?

To tell the isomers apart you can (should):

• Count total # of carbons and classify them as 1°, 2°, 3°, or 4°. (sometimes isomers give same count! but this is quick and dirty.)

• Build models (must practice!)

• Name (foolproof!).

General Nomenclature Rules.

1. Find a parent chain (longest continuous).

2. Number the main chain atoms so as to start nearer to the first branch.

3. Name substituents and assign numbers appropriate to their positions along the main chain.

4. Write a name as a single word.

5. Complex (branched) substituents are named as if they were a compound.

Details of Nomenclature Rules.

1a. Find a parent chain (longest continuous). Always try all possible permutations!

1

2

34 5

1

2

3

45

6

12

3

45

6

branched substituentsmaller chain

correct


1b. Find a parent chain (longest continuous). Maximize the number of branch points, i.e. avoid substituents that are branched (if possible).

branched substituentcorrect


2a. Number the main chain atoms so as to start nearer to the first branch.

34

5

6

7

1

22

1

34

56 7

8

8

correct


2b. Number the main chain atoms so as to start nearer to the first branch. If the substituents are at the same distances from chain ends give lower number to the substituent with the alphabetical priority of the first letter.

2

3

4

5

6 1

21 3

45 6

Note that the ethyl group will get position number 3, not the methyl.


2c. Number the main chain atoms so as to start nearer to the first branch. If there are more than two substituents, begin numbering at the end nearer to the two closest branch points.

34

5

2

23

4

56

6

1

7

78

1 8


4a. Write a name as a single word. Pay attention to hyphens and commas.

23

4

15

67

2,3-dimethyl-5-ethylheptane

1

23

45 6

3-ethyl-4-methylhexane


4b. Write a name as a single word. Branched substituents are named as a separate molecule. The numbering within the name always starts nearest to the attachment point of the complex substituent.

1

23

4

5

61

2

4-(1-methylethyl)heptane

7

2,3-dimethyl-6-(2-methylpropyl)decane

1 2 34

5

6

7

8

9

10

2

3

4

56

7

8910

6-(1,2-dimethylpropyl)-3-ethyldecane

1

drawing chemical structures condensed structures: c-h and c-c and single bonds aren't shown but...

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