Hydrogen Deficiency Index Degrees of Unsaturation

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KIM3204 STRUCTURAL IDENTIFICATION OF CHEMICAL COMPOUNDS

CHAPTER 1 (Cons)

Saturated Hydrocarbons CnH2n+2 CH4 CH3CH3 CH3CH2CH3 CH3CH2CH2CH3 CH3CH2CH2CH2CH3

GENERAL FORMULA CH4

CH3

YOU CAN DETERMINE THE NUMBER OF RINGS AND / OR DOUBLE BONDS.

FORMATION OF RINGS AND DOUBLE BONDS Notice that the general formula for the compound, , changes when a double or triple bond is present:

C2H6 C3H8 C4H10 C5H12

Adding a degree of unsaturation decreases the number of H atoms by two.

CH3 CH3 CH3 CH CH C CH3

WHAT CAN YOU LEARN FROM A MOLECULAR FORMULA ?

C9H20

CH3 branched compounds also follow the formula

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FORMATION OF RINGS AND DOUBLE BONDS -2H

C C

Index of Hydrogen Deficiency

C C

Index of Hydrogen Deficiency (IHD): the sum of the number of rings and pi bonds in a molecule.

C C

To determine IHD, compare the number of hydrogens in an unknown compound’s equivalent hydrocarbon with the number in a reference hydrocarbon of the same number of carbons and with no rings or pi bonds

H H H H

-4H

C C H H H2C H2C

CH2 CH2 H CH2

-2H

CH2 H

H2C H2C

CH2 CH2 CH2

CH2

the molecular formula of the reference hydrocarbon is CnH2n+2

Formation of each ring or double bond causes the loss of 2H.

Index of Hydrogen Deficiency Summary of how the index of hydrogen deficiency works


A double bond and ring each counts as one IHD
A triple bond counts as two IHD CALCULATION METHOD Determine the expected formula for a noncyclic, saturated compound ( CnH2n+2 ) with the same

Index of Hydrogen Deficiency Example 1 : A compound with molecular formula C6H12 C6H14 = formula corresponding alkane (hexane) C6H12 = formula of compound (1-hexene or cyclohexane) H2 = difference = 1 pair of hydrogen atoms IHD = 1

Example 2 : A compound with molecular formula C5H8

number of carbon atoms as your compound. Correct the formula for heteroatoms Subtract the actual formula of your compound

C5H12

= ( CnH2n+2 )

C5H8 H4

Index = 4/2 = 2

Two Unsaturations double bond and ring in this example

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Index of Hydrogen Deficiency

Example 3 : A compound with molecular formula C4H5N

CORRECTIONS FOR ATOMS OTHER THAN HYDROGEN

O or S -- doesn’t change H in calculated formula (Ignore)

H

C4H10

= ( CnH2n+2 )

C4H11N

+0

C-H

+O

C-H

+N,+H

add one H for N

C-O-H C 4 H5 N

N or P -- add one H to the calculated formula

+1

N

H6 C-NH2

Index = 6/2 = 3

two double bonds and ring in this example

F, Cl, Br, I -- subtract one H from calculated

formula

-1

C-H

-H,+X

C-X

The index gives the number of • double bonds or • triple bonds or • rings in a molecule Benzene

one ring and the equivalent of three double bonds gives an index of 4

PROBLEM

INDEX

A hydrocarbon has a molecular formula of C6H8. It will react with hydrogen and a palladium catalyst to give a compound of formula C6H12. Give a possible structure.

C6H14 -C6H8 H6

If index = 4, or more, expect a benzene ring

Index = 6/2 = 3

HYDROGENATION C6H8

+ 2 H2

Pd

C6H12

Hydrogenation shows only two double bonds. Therefore, there must also be a ring.

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A FEW POSSIBLE ANSWERS CH3 CH3

H3C

CH3

CH3

CH2CH3 ..... there is still work required to fully solve the problem

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