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Chemistry 2000 Lecture 17: Introduction to organic chemistry Marc R. Roussel March 1, 2018 Marc R. Roussel Introduction to organic chemistry March 1, 2018 1 / 22

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Chemistry 2000 Lecture 17:Introduction to organic chemistry

Marc R. Roussel

March 1, 2018

Marc R. Roussel Introduction to organic chemistry March 1, 2018 1 / 22

Organic chemistry

The organic chemist’s periodic table

P

NC O

Cl

Br

I

F

S

H

Marc R. Roussel Introduction to organic chemistry March 1, 2018 2 / 22

Organic chemistry

The chemistry of carbon

Carbon almost universally adopts a complete octet by forming fourtwo-electron bonds.

Carbon can form single, double or triple bonds both to itself and toother elements.

Possible (unstrained) bonding geometries:

Tetrahedral: four single bondsTrigonal planar: one double bond and two single bonds

Linear: two double bonds or one triple bond and one single bond

Marc R. Roussel Introduction to organic chemistry March 1, 2018 3 / 22

Structural diagrams

Structural diagrams

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

H

C C C

H

ClH

H

H

1-chloropropene

Carbon-hydrogen bonds are often unchanged in organic reactions, sowe 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, givingus a condensed structural formula:

CH3CH=CHCl

Marc R. Roussel Introduction to organic chemistry March 1, 2018 4 / 22

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 out thehydrogen 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 wherea line ends.Enough hydrogen atoms are assumed to be there to satisfy carbon’snormal valence of 4.

Marc R. Roussel Introduction to organic chemistry March 1, 2018 5 / 22

Structural diagrams

Structural diagramsDelocalized rings

Benzene (C6H6), and some other ring systems with delocalized πbonds, have a special representation as line-bond structures.

Because the π bonds are delocalized over the ring, we do not drawdouble bonds between specific pairs of atoms.

Instead, we draw a circle to represent the delocalized π electrons:

Marc R. Roussel Introduction to organic chemistry March 1, 2018 6 / 22

Structural diagrams

Structural diagramsLone pairs, charges, mixed structures

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

••

••

O•

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

H

•••

C

O•

Marc R. Roussel Introduction to organic chemistry March 1, 2018 7 / 22

Structural diagrams

Exercise

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

1

2 Br

3OH

4

F

Marc R. Roussel Introduction to organic chemistry March 1, 2018 8 / 22

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 iscalled a functional group.

When describing functional groups, we often use R to represent anarbitrary carbon chain (or H).

Marc R. Roussel Introduction to organic chemistry March 1, 2018 9 / 22

Functional groups

Structural feature Functional group ExampleC-H compound with noother functional groups

alkane CH3CH3

C C alkene CH2=CH2

–C≡C– alkyne CH≡CH

arene

–OH alcohol CH3OH

OH

phenolOH

R1–O–R2 ether CH3–O–CH3

Marc R. Roussel Introduction to organic chemistry March 1, 2018 10 / 22

Functional groups

Structural feature Functional group Example

C H

O

aldehydeO

HH3C C

R2R1 C

O

ketoneCH3CH3 C

O

C

O

OHcarboxylic acid

CH3 C

O

OH

C

O

O Rester

CH 3C OH

O

R1 R2

R3

N

amine CH3NH2

R1

R2

C N

O

amideNH2CH3 C

O

Carbonyl group: C=O

Marc R. Roussel Introduction to organic chemistry March 1, 2018 11 / 22

Functional groups

Exercise: Find the functional groups

Isoamyl acetate: (artificial banana flavor)CH 3O C

O

Vanillin:O CH 3

OH

CHO

Aspartame:

O

H2

H2N

CH2

C OH

C

CH3C

O

OC

O

NHCH CH

. .

:

. .

. .

: :

. .

: :

. .

. .

:

Marc R. Roussel Introduction to organic chemistry March 1, 2018 12 / 22

Isomerism

Isomerism

Isomers are compounds with the same chemical formula (sameatoms), but arranged differently.

Structural isomers differ in what is bonded to what.

Stereoisomers have identical chemical bonds, but are arranged differentlyin space.

Marc R. Roussel Introduction to organic chemistry March 1, 2018 13 / 22

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):

Marc R. Roussel Introduction to organic chemistry March 1, 2018 14 / 22

Isomerism

Exercise

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

Marc R. Roussel Introduction to organic chemistry March 1, 2018 15 / 22

Isomerism

Some types of stereoisomerism

Geometrical isomers have identical bonds, but the distances between somenonbonded atoms are different due to a differentarrangement of the bonds in space.

In compounds with rings to which two substituents are attached, thetwo substituents can be on the same side of the ring (say, both up) oron opposite sides (one up, one down).

Both on the same side: cisSubstituents on opposite sides: trans

Example: The compound has two geometrical isomers:

CH 3CH 3

H H CH 3

CH 3

H

H

cis trans

Marc R. Roussel Introduction to organic chemistry March 1, 2018 16 / 22

Isomerism

Due to the rigidity of the π bond, molecules with double bonds alsohave cis and trans isomers.

Example:

CH 3C2H 5

HH CH 3

C2H 5 H

H

cis trans

Marc R. Roussel Introduction to organic chemistry March 1, 2018 17 / 22

Isomerism

Geometrical isomers generally have different physical and chemicalproperties.

Example: Which one of the following isomers has a higher boilingpoint?

Cl

HH

Cl H

ClH

Cl

Marc R. Roussel Introduction to organic chemistry March 1, 2018 18 / 22

Isomerism

Example: Fats have the general formula

CH2

CH2

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 inarteries.

Marc R. Roussel Introduction to organic chemistry March 1, 2018 19 / 22

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 chiralmolecules rotate plane-polarized light.

In organic chemistry, enantiomerism arises most often when amolecule contains one (or more) carbon atoms attached to fournon-equivalent groups.Such a carbon atom is called a chirality centre.

R1

R2 R3

R4

R1

R2R3

R4

CC

Marc R. Roussel Introduction to organic chemistry March 1, 2018 20 / 22

Isomerism

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

Which of the following is chiral?

1CH 3

C2H 5

NH

2

Cl

Cl

3

CH3

CH3

C

OH

H

C

H

OH

Marc R. Roussel Introduction to organic chemistry March 1, 2018 21 / 22

Isomerism

Enantiomers have identical physical properties and reactivity unlessreacting with another chiral agent.

Amino acids (and thus enzymes) are chiral.

Enzymes can therefore carry out stereo-specific reactions, which isvery hard to do with ordinary chemical reagents.

Marc R. Roussel Introduction to organic chemistry March 1, 2018 22 / 22