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Chapter 2
Types of bonds

Sigma – the overlap is directly between the two nuclei
o Every bond contains one (and only one) sigma bond

Pi – the overlap is above and below the sigma bond
o These are formed by the overlap of unhybridized p orbitals
o There is one sigma bond and one pi bond in a double bond.
o There is one sigma bond and two pi bonds in a triple bond.

How many sigma and pi bonds in the following molecule?
CN
o You need to draw out all the bonds (including hydrogens which were left off) in
order to answer this.
o There are 14 sigma bonds and 3 pi bonds.
Hybridization

When orbitals hybridize, the s and some or all of the p orbitals of the outer shell
combine to form hybrid orbitals
o The p orbitals that don’t get involved remain p orbitals
__ __ __
p p p
4 sp3 orbitals
__
s
__ __ __
p p p
3 sp3 orbitals and 1 p orbital
__
s
__ __ __
p p p
2 sp orbitals and 2 p orbitals
__
s


What’s the hybridization?
o Count the charge clouds (sigma bonds and lone pairs)
 Call it #
o Hybridization is sp#-1
 So if there are three charge clouds, then the hybridization is sp3-1=sp2
What’s the electronic geometry?
o sp → linear
o sp2→ trigonal planar
o sp3 → tetrahedral


What’s the bond angle?
o sp→ 180°
o sp2 → 120°
o sp3 → 109.5°
What’s the molecular geometry?
o If all the charge clouds are bonds, then same as electronic geometry
o If 2 bonds and one lone pair, then bent

o If 3 bonds and one lone pair, then trigonal pyramidal
o If 2 bonds and 2 lone pairs, then bent
Big picture!

When you identify the hybridization of an atom in a molecule you are
identifying what type of orbitals that atom has.

Bonds form from the overlap of these hybrid orbitals.

A frequently missed type of question in this class is something like “the
carbon-oxygen sigma bond of acetone is formed from the overlap of what
orbitals?”
o Identify the hybridization of both atoms involved.
sp2
sp2
o Look to see if you are asked for the sigma or pi bond.

If you were asked for the pi bond, then it’s always the overlap
of two p orbitals.

If you were asked for the sigma bond, then it’s just the two
types of hybrid orbitals of each atom.

Bond rotation

Single bonds rotate
In this case because both the oxygen and carbon are
sp2 hybridized the sigma bond is formed from the
overlap of two sp2 orbitals.
These are two conformations of
the same molecule.

Double and triple bonds don’t
These are two different molecules.

o You would have to break the pi bonds in order to rotate around that bond.
It’s really that simple!
o If this isn’t making sense, play with your models.
Drawing in 3D

Anything coming out of the page should be drawn on a wedge and anything drawn
going into the page should be drawn on a dash.

If an atom is sp3 hybridized, make sure you draw the dashed and wedged pieces outside
the angle formed by the two flat pieces.
Wrong!
Right!
o This doesn’t make that much of a difference now, but from quiz 2 and on,
you will miss points (a lot of them) if you do this incorrectly.
Isomerism


Structural isomers and constitutional isomers are the same thing
o They have the same molecular formula, but the atoms are connected in a
different order.
o If you can’t tell whether two molecules are identical or isomers, see if you would
name them differently
 Ex. 2,4-dimethylhexane vs 2,3-dimethylhexane
Stereoisomers
o The atoms are connected to each other in the same order, but they differ in their
arrangement in space


Ex. R/S, E/Z, cis/trans
At this point in the course, you are only responsible for cis/trans
isomerism
 There are two types of cis/trans isomers
 cis/trans double bonds
o cis double bonds have both “pieces” on the same side of
the double bond
o trans double bonds have both “pieces” on opposite sides
of the double bond
o When do I have cis/trans isomerism possible?
A
C
B
D

You only have cis/trans isomerism possible when
A≠B and C≠D
no stereoisomers


cis/trans stereoisomers
Be careful to make sure the double bond is in the
same position when you are comparing two similar
structures.
 The two compounds on the right above are
stereoisomers of each other and the compound on
the left is a structural isomer of the right two.
cis/trans on rings
o When both substituents on the ring are facing the same
way (both on wedges or both on dashes), then you have a
cis isomer.
o When the two substituents on the ring or facing different
ways, then you have a trans isomer.
o Be careful when comparing two compounds to make sure
that the two substituents are still on the same carbons of
the ring.

These two are structural isomers, not
stereoisomers because the chlorines are 1,3 to
each other on the first ring and 1,2 to each other
on the second ring.
Intermolecular Forces

Boiling point is a good measure of intermolecular forces.
Boiling point (C)

-11.7
56
82
Solubility is also an expression of intermolecular forces.
118
222
o Like dissolves like.

Like in polarity.
o What will be soluble in water?
Solubility

Salts

Polar organic molecules where the polar part is not overcome by huge
nonpolar R-groups.

Ex. As the carbon chains of alcohols get longer, they become less soluble
in water and more soluble in nonpolar solvents.

0.08%
11%
miscible
With larger carbon pieces, the more branched isomer will be more
soluble in water than the less branched isomer.
OH
OH
miscible
11%
o What will be soluble in hexane?


Most organic compounds, as long as they’re not too polar.
London dispersion
o Present in all molecules
o The weakest of the attractions
o Which of the following has the lowest boiling point?

Branching lowers boiling point, so the third molecule has the lowest.

This is because more branched isomers are more compact, so the London
dispersion forces are smaller.

The unbranched chain, then, has the highest boiling point.
Boiling point (C)

69
62
50
Dipole-dipole
o The interaction between two polar molecules
o Stronger than London dispersion


Remember that polar molecules still have London dispersion forces.
Hydrogen-bonding
o Only happens in molecules where hydrogen is bonded to oxygen, nitrogen, and
fluorine.
o Remember that hydrogen-bonding compounds still experience dipole-dipole
attractions and London dispersion forces.
Classes of compounds
Classification
Alkanes
Alkenes
Alkynes
Aromatics
Alcohols
Functional group
Just carbon and hydrogen
No multiple bonds
At least one carbon-carbon double bond
At least one carbon-carbon triple bond
For now, benzene and compounds that
contain benzene rings
-OH (hydroxyl group)
Ethers
Aldehydes
R-O-R’
R
(carbonyl group)
R
R' (carbonyl group)
R
OH (carboxyl group)
Ketones
Carboxylic Acids
Acid chlorides
R
Esters
R
OR'
Amides
R
Amines
R
(amino group)
Nitriles
R-CN
You’re not responsible for naming these compounds until we cover them later.

For Test 1 of 301, we’ll have nomenclature of alkanes.

For Test 3 of 301, we’ll likely have nomenclature of alkenes.

For the Final of 301, we’ll likely have nomenclature of alkynes and alcohols.
A molecule can belong to more than one class of compound!
I can guarantee that at some point you will be asked to circle the functional groups on a large
molecule and state what class of compound each functional group makes the molecule.

Understand the difference between functional group and class of compound!

Ex. –OH is a functional group called a hydroxyl. If a molecule has that functional group, then it
is an alcohol.

You don’t need to circle the boring bits of the molecule and say “alkane.” That’s just the
default.

“Cyclic” is not a class of compound or functional group.