-Structural isomers-Naming Branched Alkanes

IsomersIsomers

Compounds with the same molecular formula Compounds with the same molecular formula but different structural arrangement are but different structural arrangement are known as structural isomers.known as structural isomers.

The more C atoms there are in the formula, The more C atoms there are in the formula, the more structural isomers there will be. the more structural isomers there will be.

4C (1) 5C(3) 6C(5) 8C(18)

C20(>303,000)

Structural IsomersStructural Isomers In order to determine if In order to determine if

something is an isomer something is an isomer determine the molecular determine the molecular formulaformula

To the right we haveTo the right we have pentane (Top)pentane (Top) 2 – methyl butane2 – methyl butane

What is the mol. Formula What is the mol. Formula for each? Are theyfor each? Are they

structural isomers?structural isomers?

C5H12 – Yes, theyAre isomers

IsomersIsomers

Isomers have different structures and Isomers have different structures and different different chemicalchemical and and physicalphysical properties. properties.

Butane 2-methylpropaneFormula C4H10 C4H10

B.P. 0 C -12 CM.P. -138 C -159 CDensity 0.622 0.604Sol. In 100ml Alcohol 1813 ml 1320 ml

Recall that the members of the group of alkanesForms a homologous series and each member ofThis series differs from the last by 1 –CH2- unit

When we draw the structural formulas of the 1st

Three members of this group there is only oneWay each can be drawn.

CH4 CH3-CH3 CH3-CH2-CH3

Drawing simple alkanesDrawing simple alkanes

Branched-chain alkanesBranched-chain alkanes

Beginning with butane, CBeginning with butane, C44HH1010, there is , there is more than 1 way to arrange the atoms more than 1 way to arrange the atoms besides one carbon after another.besides one carbon after another.

CH3-CH2-CH2-CH3 & CH3

|CH3-CH-CH3

Branched-chain alkanesBranched-chain alkanes In both butane structures we have the same In both butane structures we have the same

numbers and kind of atoms namely, Cnumbers and kind of atoms namely, C44HH1010

The general formula for each is also CThe general formula for each is also CnnHH2n+22n+2 so so each represents the alkane “Butane”each represents the alkane “Butane”

Yet there is a difference. The difference lies in what Yet there is a difference. The difference lies in what atoms are joined to what atomsatoms are joined to what atoms

CH3-CH2-CH2-CH3 & CH3

* |CH3-CH-CH3

*Are there any other ways to draw 4 carbon butane?Nope

Note that what we have drawn for butane representsReal structural changes and not just apparent changesAs a result of the rotation of a C-C single bond.

Let’s Look a pentane to see what we mean by this.

Rotation of this bondLeads to this configurationOf pentane

And rotation of thisBond leads to this

All of these structures however represent the SAME molecule!

Pentane isomersPentane isomersTo find new structural isomers of the straight chain Alkanes we need to move the point of attachment ofThe various carbon atoms.

Straight chainisomer

Remember, carbonMust have 4 bonds.No more, No less!

So how many structuralIsomers does pentaneHave?

Naming branched-chain Naming branched-chain alkanesalkanes

Find the longest continuous chain or backbone of C atoms. Find the longest continuous chain or backbone of C atoms.

c-c-c-c-c-c-c-c-c-cc-c-c-c-c-c-c-c-c-ccc c c What’s the longest chain?What’s the longest chain?

cc

The base name is derived from the number of C’s in the longest chain.The base name is derived from the number of C’s in the longest chain.

10 carbons would be decane

Naming branched-chain Naming branched-chain alkanesalkanes

Branches are added Branches are added as a prefix and are as a prefix and are named by counting named by counting the number of C the number of C atoms.atoms.

The “branch” alkane The “branch” alkane name ends in “yl.”name ends in “yl.”

MethylMethyl CHCH33——

EthylEthyl CHCH33CHCH22 – –

PropylPropyl CHCH33CHCH22CHCH22 ––

ButylButyl CHCH33CHCH22CHCH22

CHCH22 – –

c-c-c-c-c-c-c-c-c-cc-c-c-c-c-c-c-c-c-c c cc c c c

Naming branched-chain Naming branched-chain alkanesalkanes

The location of the branch (or substituent group) is shown The location of the branch (or substituent group) is shown by assigning numbers to the C’s in the backbone. by assigning numbers to the C’s in the backbone.

Number from the end that gives the lowest number for Number from the end that gives the lowest number for the first branch.the first branch. Substituent groups (branches) are listed alphabeticallySubstituent groups (branches) are listed alphabetically

There may be more than 1 of the same type of branch. There may be more than 1 of the same type of branch. Use di, tri, tetra etc. for 2, 3, and 4 Use di, tri, tetra etc. for 2, 3, and 4 Number the locations and separate the # by a comma &Number the locations and separate the # by a comma & separate the last # from the name by a dash

7-ethyl-3-methyldecane

c-c-c-c-c-c-c-c-c-cc-c-c-c-c-c-c-c-c-c c cc c c c

1 2 3 4 5 6 7 8 9 10

H

H–C–H

H H

H–C–C–C–H

H H

H-C-H|

H

Longest continuous chain has 3carbon atoms – propane.

2 Branches each have 1 carbon – dimethyl.Branches have to be at C-2. WHY??

(Only give the number if necessary.)

Dimethylpropane or

C5H12 or CH3C(CH3)2CH3

Example:

Longest continuous chain has 4carbon atoms – butane.

2 Branches each have 1 carbon – dimethyl.Branches are at C-2. WHY??

2,2-Dimethylbutane or

C6H14 or CH3C(CH3)2CH2CH3

CH3 | H3C-C-CH2-CH3

| CH3

Name this compound

H

H – C – H

H H HH – C – C – C – C – H H H H–C–H H–C–H H H

Longest continuous chain has 6 carbon atoms: hexaneBranch is 1 carbon long: methylBranch is located at C-3

3-methylhexane or

C7H16 or

CH3CH2CH2CH(CH3)CH2CH3 or

CH3(CH2)2CH(CH3)CH2CH3

Name this compound:

More Naming ProblemsMore Naming Problems

CH

H3C CH

CH2

CH2

CH2

CH

CH3

CH3

CH3 CH3

CH

H3C CH2

CH3

CH3

Problems:Problems:

Draw the following:Draw the following:

2 – methylpropane2 – methylpropane 3 – ethyl – 4 - 3 – ethyl – 4 -

methylnonanemethylnonane 2,2 – 2,2 –

dimethylpropanedimethylpropane 2,3,4 – 2,3,4 –

trimethyldecanetrimethyldecane

What name does this compound have?