chemistry 2000 lecture 17: introduction to organic...
Post on 25-May-2018
220 Views
Preview:
TRANSCRIPT
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
top related