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