Basics of Organic Chemistry  Introduction to Organic Synthesis      

How to make alkanes and cycloalkanes? Hydrogenation of alkenes and alkynes Homogeneous and heterogeneous catalysis Index of hydrogen deficiency Hydrogenation and bromination Hydrogenation and trans-fat

1

Synthesis of Alkanes and Cycloalkanes  Hydrogenation of Alkenes and Alkynes

13.4 (Ed.1)

12.3 (Ed. 2)

2

Synthesis of Alkanes and Cycloalkanes  Example

13.4 (Ed.1)

12.3 (Ed. 2)

3

Hydrogenation of Alkenes  Hydrogen adds to alkenes in the presence of

metal catalysts  Heterogeneous catalysts: finely divided insoluble platinum, palladium or nickel catalysts  Homogeneous catalysts: catalyst (typically rhodium or ruthenium based) is soluble in the reaction medium  Wilkinson’s catalyst is Rh[(C6H5)3P]3Cl

 This process is called a reduction or

hydrogenation  An unsaturated compound becomes a saturated (with

hydrogen) compound

13.4 (Ed.1)

12.3 (Ed. 2)

4

Hydrogenation of Alkenes: Examples

Heterogeneous catalysts

saturated

unsaturated Homogeneous catalyst 13.4 (Ed.1)

12.3 (Ed. 2)

5

Hydrogenation: The Function of the Catalyst  The catalyst provides a new reaction pathway with lower

DG‡ values

Chapter 7

6

Heterogeneous Catalysis  In heterogeneous catalysis the hydrogen and

alkene adsorb to the catalyst surface and then a step-wise formation of C-H bonds occurs

7

Heterogeneous Catalysis  Both hydrogens add to the same face of the

alkene (a syn addition)  Addition to opposite faces of the double bond is called

anti addition

8

Hydrogenation of Alkynes  Reaction of hydrogen using regular metal catalysts results in formation of the alkane

Special catalysts are needed to stop the reaction from going further. 9

An internal alkyne will yield a cis double bond

Heterogeneous catalysis Ni2B (aka P-2) is a special catalyst that only leads to the alkene product. 10

Matching Game  Match the catalyst with the type of reactions it is used for.

Rh(Ph3P)3Cl

Homogeneous hydrogenation

Pt

Heterogeneous hydrogenation

11

The Index of Hydrogen Deficiency (IHD)  The number of pairs of hydrogen atoms that must

be subtracted from the molecular formula of the corresponding alkane to give the molecular formula of the compound under consideration  A compound with the general molecular formula CnH2n

will have either a double bond or a ring  A compound with general formula CnH2n-2 can have a triple bond, two double bonds, a double bond and a ring or two rings  A compound with general formula CnH2n-4 can have a triple bond and a double bond, three double bonds, a double bond and two rings, etc.

12

Index of Hydrogen Deficiency A mysterious liquid has been found to have a formula of C6H12. Is it a saturated, unsaturated, or cyclic compound?

It has either one double bond, or is a cyclic compound. 13

Hydrogenation for Identification  Hydrogenation allows one to distinguish a compound

with a double bond from one with a ring

14

 In-Class Practice: Squalene has the molecular formula C30H50 and

has no triple bonds.  A) What is the index of hydrogen deficiency of squalene?  B) Squalene undergoes catalytic hydrogenation to yield a compound with the molecular formula C30H62. How many double bonds does a molecule of squalene have?  C) How many rings?  Answer:

 A) Saturated squalene would have a formula of C30H62. So the

index of hydrogen deficiency is 62-50=12/2=6  B) Hydrogenation saturated squalene to an alkane formula. Therefore squalene should have 6 double bonds.  C) No rings.

1 (6E,10E,14E,18Z)-2,6,10,15,19,23-He xamethyl-tetracosa-2,6,10,14,18,22Chapter 7 hexaene

Squalene

15

Bromination for Identification Another useful experiment: identify alkene and alkyne compounds with a bromine (Br2/CCl4) solution.  Qualitatively:

The red-brown color of Br2 will pale or disappear instantly in the presence of alkenes or alkynes (dark, room temperature).  Quantitatively:

1 mol of Br2 will be needed to add to 1 double bond.

13.4 (Ed.1)

12.3 (Ed. 2)

16

Bromination for Identification Addition reaction of alkenes and alkynes dark, CCl4 Br

Br2

Br Cl

+

Cl

2Cl2

dark, CCl4

4-decyne

Cl

Cl

4,4,5,5,-tetrachlorodecane 13.4 (Ed.1)

12.3 (Ed. 2)

17

Hydrogenation and Trans Fat  Most trans fats consumed today are industrially created

through partial hydrogenation hydrogenation of plant oils and animal fats — a chemical process developed in the 1900s and first commercialized as Crisco Crisco in 1909.

x

Partial O

* n

OH

*

O

*

* trans-fat

OH

Hydrogenation

cis-fat

x

O

*

* saturated-fat 18.2 (Ed.1)

17.2 (Ed. 2)

OH 18

Trans fat on food labels

Chapter 7

19

Fats on food labels

Find the hidden trans-fat

So what are all these fats?

They are fatty acids.

Do the simple math.

18.2 (Ed.1)

17.2 (Ed. 2)

20

SATURATED FATTY ACIDS H H H H H H H H H H H H H H H H H O H C C C C C C C C C C C C C C C C C C O H H H H H H H H H H H H H H H H H H

carboxylic acid end

methyl end

O

H

C O

H C H

H

carboxylic acid end

methyl end 18.2 (Ed.1)

17.2 (Ed. 2)

21

SATURATED FATTY ACIDS O

H

C O

H C H

C12:0 Lauric Acid (Coconut)

H H

O C O

H C

O C O

H C

H

C18:0 Stearic Acid (Animal fat) O

H

C O

H C H

H

C16:0 Palmitic Acid (Palm)

H H

H

C14:0 Myristic Acid (Nutmeg)

H H

O C O

H C

H

C20:0 Acid ChapterArachidic 7 (Vegetable and Fish Oil)

H 22

MONOUNSATURATED FATTY ACIDS H H H H H H H H

H H H H H H H O

H C C C C C C C C C C C C C C C C C C O H H H H H H H H H H H H H H H H H H

carboxylic acid end

methyl end

O

H

C O H

H C H

carboxylic acid end

methyl end 18.2 (Ed.1)

17.2 (Ed. 2)

23

MONOUNSATURATED FATTY ACIDS O

H

C O H

H C H

C16:1

Palmitoleic Acid (Butter) O

H

C O H

H C H

C18:1 Oleic Acid (Olives, Corn)

18.2 (Ed.1)

17.2 (Ed. 2)

24

POLYUNSATURATED FATTY ACIDS

H O

H C

C O H

H

methyl end

carboxylic acid end O

H

C O H

H C H

carboxylic acid end

methyl end 18.2 (Ed.1)

17.2 (Ed. 2)

25

The “OMEGA” System of Numbering H 1 H C H

3 2

5 4

methyl end

O C O H

6

OMEGA-6 FATTY ACID

O

H 2 H C 1 H

methyl end

carboxylic acid end

C O H

3

OMEGA-3 FATTY ACID

18.2 (Ed.1)

17.2 (Ed. 2)

carboxylic acid end 26

OMEGA-6 FATTY ACIDS H 1 H C

3 2

H

5 4

O C O H

6

Linoleic Acid

H 1 H C H

C18:2

5

3 2

Essential!

4

O C O H

6

Arachidonic Acid

18.2 (Ed.1)

17.2 (Ed. 2)

C20:4

27

OMEGA-3 FATTY ACIDS

O

H 2 H C 1 H

Essential!

C O H

3

Linolenic

C18:3

H 2

O

H C 1 H

C O H

3

Eicosapentaenoic Acid (EPA)

C20:5 O

H 2 H C1 H

C O H

3

Docosahexaenoic Acid (DHA) 18.2 (Ed.1)

17.2 (Ed. 2)

C22:6 28

Comparison of dietary fats

Chapter 7

29

Food industry

more saturation = more solid more unsaturated = more fluid

30

Hydrogenation of Fatty Acids essential O C O H

Linolenic Acid

C18:3

Hydrogen (H2) + catalyst

unwanted

(3 molecules needed) O C O H

Stearic Acid 18.4 (Ed.1)

C18:0

17.4 (Ed. 2)

31

Partial Hydrogenation of Fatty Acids O C

O

H

Hydrogen (H2) + catalyst O C

O

H

H

H

H H Cis double bond

Trans double bond

18.4 (Ed.1)

17.4 (Ed. 2)

32

Why nature prefers cis-fatty acids? Why trans-fat behaves similarly to cis-fat? O C O H

Structure, structure, structure. O C O H

O C O H 33