1CHEM 3013 ORGANIC CHEMISTRY I LECTURE NOTES

CHAPTER 3

1 . Alkanes

Hydrocarbons contain only carbon and hydrogen. Alkanes (also called paraffins) arehydrocarbons which contain only single, sigma bonds between the carbons that make-up thebackbone.

CC

H

SP3-SP3 sigma bond

H

HHH

H

SP3-1S sigma bond

normal bonds: in plane of pagedark wedge: out of pagedashed wedge: behind page

Bonding in ethane

2 . Normal alkanes

Normal alkanes (n-alkanes) are linear, unbranched alkanes. General Molecular Formula CnH2n+2. Members of a homologous series have predictable chemical and physical properties(mp, bp, density, reactivity). The first four straight chain alkanes have non-systematic names.Thereafter, the alkanes are named using Greek number prefixes (penta, hexa, hepta, octa,nona, deca...) followed by the suffix -ane

methane CH4ethane CH3CH3propane CH3CH2CH3butane CH3(CH2)2CH3pentane CH3(CH2)3CH3hexane CH3(CH2)4CH3heptane CH3(CH2)5CH3octane CH3(CH2)6CH3nonane CH3(CH2)7CH3decane CH3(CH2)8CH3

......general CH3(CH2)nCH3

A homologous series with(CH2)n increasing by onemethylene unit for eachsucceeding homolog.

CnH2n+2

The n-alkanes

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3 . Isomers

Compounds with the same molecular formulas, but different structures are called isomers.Isomers have different chemical and physical properties. Compounds which differ in their carbon connectivity are called structural isomers or constitutional isomers . The number of possibleconstitutional isomers increases dramatically as the number of carbon atoms increases.

Formula Number of isomers

CH4 1C2H6 1 C3H8 1C4H10 2C5H12 3C6H14 5C7H16 9C8H18 18C9H20 35C10H22 75..C15H32 4,347..C40H82 62,491,178,805,831

Isomers of n-alkanes

H3C

H2C

CH2

H2C

CH3 H3CCH

CH2

CH3H3C

CCH3

CH3 CH3

CH3

n-pentane isopentane neopentane

Non-systematic common names

The structural isomers of C5H12

4. Alkyl groups

If a hydrogen atom is removed form an alkane, the structure that remains is called an alkylgroup. Alkyl groups are named by replacing the -ane ending of the parent alkane with -yl.Removal of a hydrogen from methane generates a methyl group, removal from ethane generates anethyl group. Likewise, removal of a hydrogen from the terminal carbon of a straight-chain alkanegenerates a n-alkyl group. Removal of an internal hydrogen generates a branched alkyl group.

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CH4

CH3CH3

CH3CH2CH3

CH3CH2CH2CH3

CH3

CH3CH2

CH3CHCH3 CH3CH2CH2

CH3CH2CHCH3 CH3CH2CH2CH2

Methane methyl group

Ethane ethyl group

Propane isopropyl group propyl group

Butane sec-butyl group butyl group

Alkyl groups

6. Alkane nomenclature

The IUPAC system divides a chemical name into three parts: prefix, parent, and suffix.The parent tells how many C atoms are in the main chain, the suffix identifies the functional groupspresent in the molecule, and the prefix tells their position along the main chain.

Steps for naming complex, branched-chain alkanes

i. Select the longest chain of carbon atoms and give the compound the appropriate base name. (See Table 3.4, page 65.) If two or more longest chains have the same number of carbons, select the one having the most groups .

ii. Number the longest chain starting from the end nearest a branch.

iii. Give all side chains a group name by dropping the "ane" of the name corresponding to the number of carbons in the side chain and add the new suffix of "yl." All side chains are numbered starting with the carbon attached to the base (longest) chain.

iv. Precede each group name with a prefix which corresponds to the number of times the group occurs along the chain. (di, tri, tetra, penta, etc.)

v. Place each group name, including its prefix, in front of the base name in alphabetical order. Use the first letter of the group name, but not the prefix, to determine alphabetical order.

vi. Place the number of the carbon, on which each group is attached, before the group name. If a group occurs more than once on a chain, all the locations of that

group must be placed before the group name.

vii. Separate numbers from numbers with commas and numbers from words with dashes. Your final name will be one continuous word.

Naming Complex Groups

Complex groups are side chains which have groups branching off the side chain.

i. Find the longest chain in the group, starting with the point of attachment on the base chain.

ii. Number the group chain starting with the point of attachment on the base chain.

4iii. Name all groups along the group chain according to the rules iii-vii above.

iv. Enclose the entire group name, in parenthesis.v. Precede the parenthetical statement with the number of the carbon, on the base chain, on which the group is attached.

1

2

3

4

5

6

7

1 23

4

5

6

7

1

2

3

4

56

78

1 2

34

5

6

7

8

6-ethylheptane

2-ethylheptane

6-methyloctane

incorrect names

3-methyloctane Correct Name

not longest chain andnot lowest number

not longest chain

not lowest number

Findingthe longest chain and lowest position number

4 254

5 81

7 18

4,4,5,7-tetramethyloctane 2,4,5,5-tetramethyloctane

Incorrect Correct!

Lowest number to groups along the principal chain

7 . Carbon Types

Carbons in an organic compound are classified as to their structural type in the molecule.The structural type is determined by how many other carbons the carbon under consideration isbonded to. Primary carbons, (1˚), are bonded to one other carbon. Secondary carbons,(2˚) are bonded to two carbons. Tertiary carbons,(3˚), are bonded to three others.Quaternary carbons,(4˚), are bonded to four others.

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H3C

H2C

CHCH

CH2

CH3

CH3

CH3C

CH3

CH3

p

p

p

p

pp

Sst

t

q

p = primary: ( bonded to 1 C)s = secondary: ( bonded to 2 C)t = tertiary: ( bonded to 3 C)q = quaternary: (bondedto 4 C)

Carbon Types

8 . Cycloalkanes

Alkanes formed into rings are called cycloalkanes, they have the general formulaCnH2n. As a short hand, cycloalkanes are represented by polygons in skeletal drawings. Thesmallest ring size possible is a three membered ring called cyclopropane . Although large rings arepossible, the most common ring sizes are 3-,4-,5-,6-, 7- and 8-membered rings. Larger ring sizesare less common.

9 . Cycloalkane NomenclatureCycloalkanes are named by rules similar to those used for alkanes. The prefix Cyclo- is

used before the alkane name corresponding to the number of carbons in the ring. For example,consider a 7-membered ring...a seven carbon alkane is heptane....thus a 7-membered cycloalkaneis cycloheptane

Steps for naming substituted cycloalkanes .

i. Use the cycloalkane name as the parent name, i.e alkylcycloalkanes , unless an alkylside chain has more carbons than the ring...then it becomes a cycloalkylalkane

ii. When more than one substituent is present, start at a point of attachment so as to give the lowest order.iii. When two or more different substituents are present, number by alphabetical priority.iv. Treat halogens like alkyl groups.

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cyclohexane ethylcyclopentane see rule i.

2-cyclobutylheptane see rule i.

1,2-dimethyl-4-ethylcycloheptane not1-ethyl-3,4-dimethylcycloheptane see rule ii.

1-ethyl-2-isobutylcyclobutane not1-isobutyl-2-ethylcyclobutane see rule iii.

Cycloalkane Nomenclature

10. Cis-Trans Stereoisomerism in Cycloalkanes

The lack of flexibility in cycloalkanes (complete rotation around C-C bonds is prevented)has substantial consequences in that cycloalkanes have two sides, a "top" and a "bottom" side.This leads to two distinct structures when the cycloalkane is substituted with various groups. Thisis best illustrated in 1,2-dimethylcyclopropane. Note that the three carbons in cyclopropane lie inthe same plane (remember that you need three points to define a plane). One can imagine onestructure in which both methyls are on the same side of the ring plane, and another structure inwhich they are on opposite sides of the plane of the ring. The connectivity of the methyls in thetwo different structures is the same. However the o rientation in space of the two different methylsis different. These two structures are STEREOISOMERS - compounds which have the same order of connectivi ty, but different three-dimensional geometry . We define the two different three-dimensional orientations by using the prefix cis- (Latin for in the same side) or trans- (Latin;across).

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H

H

CH3

H

H3C

H H

H

H

CH3

H3C

H

CH3

H

H

CH3

H

H

H

H

CH3

CH3

H

H

H

CH3

H

H

H

H3C

CH3

CH3

cis-1,2-dimethylcyclopropane(same sides of rintg plane

trans-1,2-dimethylcyclopropane(opposite sides of ring plane)

Stereoisomers - same connections but different 3-D geometry

trans-1,2-dimethylcyclohexane

Can be represented

as

Solid Wedge:(above plane)

Dashed Wedge:(below plane)

cis-1,3-dimethylcyclohexane

Can be represented

as

Dashed Wedge:(below plane)

Dashed Wedge:(below plane)

Cis-Trans Stereoisomerism