Chemistry 2000 Slide Set 17: Introduction to organic chemistry
Transcript of Chemistry 2000 Slide Set 17: Introduction to organic chemistry
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
NC 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 formingfour two-electron bonds.
Carbon can form single, double or triple bonds both to itselfand to other 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 onesingle bond
Introduction to organic chemistry
Structural diagrams
Structural diagramsWe 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 organicreactions, 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 andhydrogen.
As noted above, the hydrogens often don’t participate inreactions.
We can simply draw lines for the bonds between carbonatoms, leaving out the carbon atoms themselves, and leavingout 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 orwhere a line ends.Enough hydrogen atoms are assumed to be there to satisfycarbon’s normal valence of 4.
Introduction to organic chemistry
Structural diagrams
Structural diagramsDelocalized rings
Benzene (C6H6), and some other ring systems with delocalizedπ bonds, have a special representation as line-bond structures.
Because the π bonds are delocalized over the ring, we do notdraw double bonds between specific pairs of atoms.
Instead, we draw a circle to represent the delocalized πelectrons:
Introduction to organic chemistry
Structural diagrams
Structural diagramsLone 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 shouldbe 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
ExerciseFor each of the following line-bond structures, draw a completeLewis diagram.
C C
H
HH
H
Br
OH
F
Introduction to organic chemistry
Functional groups
Functional groups
Compounds with common structural features (bondingarrangements, common groupings of atoms) tend to reactsimilarly.
In organic chemistry, a structural feature found in manymolecules is called a functional group.
When describing functional groups, we often use R torepresent an arbitrary carbon chain (or H).
Introduction to organic chemistry
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
Introduction to organic chemistry
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
Introduction to organic chemistry
Functional groups
Exercise: Find the functional groups
Isoamyl acetate: (artificial banana flavor)CH 3O C
O
Not counting the “boring” alkane: ester
Vanillin:O CH 3
OH
CHO
phenol, ether, aldehyde
Aspartame:
O
H2
H2N
CH2
C OH
C
CH3C
O
OC
O
NHCH CH
. .
:
. .
. .
: :
. .
: :
. .
. .
:
(methyl ester of the as-partic acid-phenylalaninedipeptide)
amine, carboxylic acid, amide, arene, ester
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 arrangeddifferently 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 (meltingpoints, etc.).
Example: Structural isomers of hexane (C6H14):
Introduction to organic chemistry
Isomerism
Exercise
Draw as many structural isomers for the formula C5H10 as you canthink of.
Introduction to organic chemistry
Isomerism
Some types of stereoisomerism
Geometrical isomers have identical bonds, but the distancesbetween some nonbonded atoms are different due toa different arrangement of the bonds in space.
In compounds with rings to which two substituents areattached, the two substituents can be on the same side of thering (say, both up) or on 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
Introduction to organic chemistry
Isomerism
Due to the rigidity of the π bond, molecules with doublebonds also have cis and trans isomers.
Example:
CH 3C2H 5
HH CH 3
C2H 5 H
H
cis trans
Introduction to organic chemistry
Isomerism
Geometrical isomers generally have different physical andchemical properties.
Example: Which one of the following isomers has a higherboiling point?
Cl
HH
Cl H
ClH
Cl
Introduction to organic chemistry
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 beunsaturated.
Natural fats always have cis double-bond geometries.
Trans fats pack together better, so they tend to be depositedin arteries.
Introduction to organic chemistry
Isomerism
Enantiomers are non-superimposable mirror images (like yourhands).
A compound that has an enantiomer is said to be chiral.
Enantiomerism is also known as optical isomerism becausechiral molecules rotate plane-polarized light.
In organic chemistry, enantiomerism arises most often when amolecule contains one (or more) carbon atoms attached tofour non-equivalent groups.Such a carbon atom is called a chirality centre.
R1
R2 R3
R4
R1
R2R3
R4
CC
Introduction to organic chemistry
Isomerism
Checking whether a molecule is chiral
Draw mirror image:
C
CH3
H
Cl
F
CH
3C
F
Cl
H
Rotate the mirror image to place it in as similar a position tothe original as possible, then compare:
C
CH3
H
Cl
F
C
H3
Cl
FCH
Introduction to organic chemistry
Isomerism
Checking whether a molecule is chiral
Note that the ultimate criterion for chirality is whether amolecule and its mirror image can be superimposed, not thepresence 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 reactivityunless 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.