Chemistry 2000 Slide Set 17: Introduction to organic chemistry Marc - - PowerPoint PPT Presentation

Introduction to organic chemistry

Chemistry 2000 Slide Set 17: Introduction to organic chemistry

Marc R. Roussel March 14, 2020

Introduction to organic chemistry Organic chemistry

The organic chemist’s periodic table

P N C O Cl Br I F S H

Introduction to organic chemistry Organic chemistry

The chemistry of carbon

Carbon almost universally adopts a complete octet by forming four two-electron bonds. Carbon can form single, double or triple bonds both to itself and to other elements. Possible (unstrained) bonding geometries: Tetrahedral: four single bonds Trigonal planar: one double bond and two single bonds Linear: two double bonds or one triple bond and one single bond

Introduction to organic chemistry Structural diagrams

Structural diagrams

We can draw a (more or less) complete Lewis diagram:

H C C C H Cl H H H

1-chloropropene Carbon-hydrogen bonds are often unchanged in organic reactions, so we often think in terms of CHn groups: CH3–CH=CH–Cl This is a condensed structure. Most bonds are single bonds, so we often leave those out too, giving us a condensed structural formula: CH3CH=CHCl

Introduction to organic chemistry Structural diagrams

CH3CH=CHCl Most organic compounds are mostly made of carbon and hydrogen. As noted above, the hydrogens often don’t participate in reactions. We can simply draw lines for the bonds between carbon atoms, leaving out the carbon atoms themselves, and leaving

• ut the hydrogen atoms attached to carbon atoms altogether:

Cl

This is a line-bond structure. Carbon atoms are assumed to be found where two lines join or where a line ends. Enough hydrogen atoms are assumed to be there to satisfy carbon’s normal valence of 4.

Introduction to organic chemistry Structural diagrams

Structural diagrams

Delocalized rings

Benzene (C6H6), and some other ring systems with delocalized π bonds, have a special representation as line-bond structures. Because the π bonds are delocalized over the ring, we do not draw double bonds between specific pairs of atoms. Instead, we draw a circle to represent the delocalized π electrons:

Introduction to organic chemistry Structural diagrams

Structural diagrams

Lone pairs, charges, mixed structures

Despite the highly simplified nature of line-bond structures, you should think of them as shorthand for a Lewis diagram. Important lone pairs and all non-zero formal charges should be placed on these diagrams.

• O•−

When we want to emphasize a particular part of the structure, it is also possible to mix the line-bond and Lewis formalisms.

H

• C

O•

Introduction to organic chemistry Structural diagrams

Exercise

For each of the following line-bond structures, draw a complete Lewis diagram.

Br OH F

Introduction to organic chemistry Functional groups

Functional groups

Compounds with common structural features (bonding arrangements, common groupings of atoms) tend to react similarly. In organic chemistry, a structural feature found in many molecules is called a functional group. When describing functional groups, we often use R to represent an arbitrary carbon chain (or H).

Introduction to organic chemistry Functional groups

Structural feature Functional group Example C-H compound with no

• ther functional groups

alkane CH3CH3

C C

alkene CH2=CH2 –C≡C– alkyne CH≡CH arene –OH alcohol CH3OH

OH

phenol

OH

R1–O–R2 ether CH3–O–CH3

Introduction to organic chemistry Functional groups

Structural feature Functional group Example

C H O

aldehyde

O H H 3 C C R2 R1 C O

ketone

CH 3 CH 3 C O C O OH

carboxylic acid

CH 3 C O OH C O O R

ester

CH 3 C O H O R1 R2 R3 N

amine CH3NH2

R1 R2 C N O

amide

NH 2 CH 3 C O

Carbonyl group: C=O

Introduction to organic chemistry Functional groups

Exercise: Find the functional groups

Isoamyl acetate: (artificial banana flavor)

CH 3 O C O

Vanillin:

O CH 3 OH CH O

Aspartame:

O H 2 H 2N CH 2 C OH C CH 3 C O O C O NH CH CH . . : . . . . : : . . : : . . . . :

(methyl ester of the as- partic acid-phenylalanine dipeptide)

Introduction to organic chemistry Isomerism

Isomerism

Isomers are compounds with the same chemical formula (same atoms), but arranged differently. Structural isomers differ in what is bonded to what. Stereoisomers have identical chemical bonds, but are arranged differently in space.

Introduction to organic chemistry Isomerism

Structural isomerism

Organic compounds typically have many structural isomers, particularly so for larger compounds with many atoms. Structural isomers have different physical properties (melting points, etc.). Example: Structural isomers of hexane (C6H14):

Introduction to organic chemistry Isomerism

Exercise

Draw as many structural isomers for the formula C5H10 as you can think of.

Introduction to organic chemistry Isomerism

Some types of stereoisomerism

Geometrical isomers have identical bonds, but the distances between some nonbonded atoms are different due to a different arrangement of the bonds in space. In compounds with rings to which two substituents are attached, the two substituents can be on the same side of the ring (say, both up) or on opposite sides (one up, one down). Both on the same side: cis Substituents on opposite sides: trans Example: The compound has two geometrical isomers:

CH 3 CH 3 H H CH 3 CH 3 H H

cis trans

Introduction to organic chemistry Isomerism

Due to the rigidity of the π bond, molecules with double bonds also have cis and trans isomers. Example:

CH 3 C2H 5 H H CH 3 C2H 5 H H

cis trans

Introduction to organic chemistry Isomerism

Geometrical isomers generally have different physical and chemical properties. Example: Which one of the following isomers has a higher boiling point?

Cl H H Cl H Cl H Cl

Introduction to organic chemistry Isomerism

Example: Fats have the general formula

CH 2 CH 2 R1 R2 R3 CH O C O C O C O O O

Fats whose R groups contain double bonds are said to be unsaturated. Natural fats always have cis double-bond geometries. Trans fats pack together better, so they tend to be deposited in arteries.

Introduction to organic chemistry Isomerism

Enantiomers are non-superimposable mirror images (like your hands). A compound that has an enantiomer is said to be chiral. Enantiomerism is also known as optical isomerism because chiral molecules rotate plane-polarized light. In organic chemistry, enantiomerism arises most often when a molecule contains one (or more) carbon atoms attached to four non-equivalent groups. Such a carbon atom is called a chirality centre. R1 R2 R3 R4 R1 R2 R3 R4 C C

Introduction to organic chemistry Isomerism

Checking whether a molecule is chiral

Draw mirror image:

C CH 3 H Cl F C H

3C

F Cl H

Rotate the mirror image to place it in as similar a position to the original as possible, then compare:

C CH 3 H Cl F C H

3

Cl F CH

Introduction to organic chemistry Isomerism

Checking whether a molecule is chiral

Note that the ultimate criterion for chirality is whether a molecule and its mirror image can be superimposed, not the presence of a chirality centre.

Introduction to organic chemistry Isomerism

Some exercises

Which of the following are chiral?

1 2 3

Introduction to organic chemistry Isomerism

Enantiomers have identical physical properties and reactivity unless reacting with another chiral agent. Amino acids (and thus enzymes) are chiral. Enzymes can therefore carry out stereo-specific reactions, which is very hard to do with ordinary chemical reagents.