Transcript PowerPoint Presentation - Chem 101/lecture 1-2

Topic 5B
Bonding in carbon compounds
3
sp
9
hybridization
• This is the reason why carbon is tetrahedral in many
compounds
• By hybridization of its valence atomic orbitals, carbon
can bond in a variety of ways
First look at the normal electronic configuration of carbon:
ml = -1
E
0
1
px
py
pz
l=1 2p
l=0
n=2
2s
Valence shell 2s2 2p2
s
3
sp
hybridization
• Promote one 2s electron into the vacant p-orbital.
• Combine (mix) all four orbitals to give four hybrid
orbitals of equivalent energy:
EE
2p
2p
2sp3
2s
2s
9
3
sp
hybridization
9
• Each sp3 hybrid orbital has 25% “s” and 75% “p”
character
• Each sp3 hybrid orbital looks like a distorted dumbell:
+
2s
2p
s p3 h ybri d
sp3 Hybridization
Animation
9
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The best arrangement of orbitals is a tetrahedral
geometry making angles of 109°
10
Tetrahedral bonding
• Each sp3 hybrid orbital has one electron and can form
a strong covalent bond with another atom, eg methane
formation with four hydrogens:
H
H
H
C
C
H
C
H
H
H
H
H
methane
H
H 109.5°
H
Sigma () bonds
• The H 1s and carbon sp3 hybrid orbitals are no longer
separate entities and combine to form a sigma ()
bonding molecular orbital.
• These bonds are 109.5° apart.
H
C
H
109.5°
H
H
10
Sigma () bond
formation
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10
Other representations
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Space Filling
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10
Other representations
Space Filling
Potential Energy
Surface
10
C–C bond formation
in Ethane
H
H
H
C
C
H
H
11
H
Ethane
Sigma () bonds can be formed between two carbons by
overlapping two sp3 hybrid orbitals.
H
H
C
H
H
H
+
C
H
H
H
H
C
H
C
H
H
sp3 - sp3 bond
between carbons
Ethane
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11
Space filling
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Ethane
Ethane can spin about the C—C bond
There is nearly free rotation:
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11
Propane
11
Propane is formed by covalent bonding to two
other carbons and eight hydrogens.
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Propane
Propane can rotate about both C—C bonds
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11
Butane
11
7
Butane is formed by covalent bonding between
four carbons and ten hydrogens.
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Space filling
Butane
11
7
Butane can rotate about all three C—C bonds
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Bonding to other atoms
12
• Alcohols are formed between sp3 hybridised
carbon and oxygen:
H
H
C
H
H
H
+
O
H
H
C
O
H
sp 3 - sp 3 valence bond
between carbon and
oxygen giving an alcohol
Bonding to other atoms
12
• Amines are formed between sp3 hybridised
carbon and nitrogen:
H
H
C
H
H
H
+
N
H
H
H
C
N
H
H
sp 3 - sp 3 valence bond
between carbon and
nitrogen giving an amine
sp2 Hybridization
Double bond formation
13
• Carbon can form double bonds with itself and other
heteroatoms.
• This requires sp2 hybridization of its valence atomic
orbitals.
• Carbon is sp2 hybridized in:
H
H
C
H
H
C
H
Ethene
(carbon sp2)
C
O
H
Formaldehyde
(carbon, oxygen sp2)
2
sp
13
Hybridization
• Promote one 2s electron into the vacant p-orbital.
• Combine (mix) the 2s, 2px and 2py orbitals to give three
hybrid orbitals of equivalent energy
• The 2pz orbital is unaltered.
EE
x
2p
y
z
2p z
2p
2s
2s
combine
2sp 2
2
sp
Hybridization
13
• Only the 2px and 2py combine with the 2s orbital.
• The three hybrid orbitals make angles of 120° to
minimise electron repulsion between them.
120°
2s
2p y
2p x
120°
120°
3 sp 2 hybrid
orbitals
Trigonal planar carbon
13
• There are four electrons — one in each orbital
• Note that the 2pz orbital is unchanged and
perpendicular to the plane of the hybrid system.
2
sp hybrid
C2pz
120°
sp2 hybrid
120°
2
sp hybrid
120°
An sp2 hybridised carbon
atom.
Pi () bonding
Ethylene
• Two sp2 carbons can form a covalentbond.
• Other hybrid orbitals covalently bond to four
hydrogens.
C2pz
C2pz
13
Pi () bonding
Ethene
14
• Less efficient sideways overlap of the pz orbitals gives a
second C—C bond — a pi () bond.
• Both clouds (shown in green and blue) are part of the
same -bonding orbital.
C2pz
C2pz
H
CH  bonds
H
H
H
CC  bond
CH  bonds
H
H
H
H
CC bond
Pi () bonding
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14
Pi () bonding
(theoretical approach)
• Overlap of two C2pz
atomic orbitals forms
two pi molecular
orbitals,  (lower in
energy) and * (higher
in energy).
• The electrons in C2pz
orbitals are stabilised
by occupying the lower
energyorbital. p
H
14
H
One *-molecular
H orbital
H
E
*
C2pz
C2pz

H
H
H
H
One  -molecular
orbital
Ethylene
Because each carbon is trigonal planar, ethylene is a flat
molecule with thickness due to the pi-electrons.
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14
Ethylene
The pi-bond restricts rotation about the C=C bond.
A little twisting is possible but it is essentially rigid.
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14
15
Ethylene
Geometry of ethylene:
121Þ H
H
C
C
118Þ
H 134 pm H
CH3 CH3
154 pm
Other double bonded systems:
H
H
C
O
H
Formaldehyde
C N
H
H
An imine
sp Hybridization
Alkyne formation
15
• Carbon can form triple bonds with itself and with other
heteroatoms (eg in H—C.
• This requires sp hybridization of its valence atomic
orbitals.
• Carbon is sp hybridized in ethyne, also called acetylene:
H
C
C
Ethyne
(carbon sp)
H
15
sp Hybridization
• Promote one 2s electron into the vacant p-orbital.
• Combine (mix) the 2s and 2px orbitals to give two
hybrid orbitals of equivalent energy
• The 2py and 2pz orbital are unaltered.
E
E
x
2p
y
z
2py
2p
2s
2s
combine
2sp
2pz
sp Hybridization
• Only the 2px combines with the 2s orbital.
• The two hybrid orbitals make angles of 180° to
minimise electron repulsion between them.
180°
2s
2px
Two colinear sp hybrid
orbitals
15
sp hybridised carbon
• The two hybrid orbitals are semi-occupied
• Note that the 2pz and 2py orbitals are unchanged and
perpendicular to the plane of the hybrid system.
C2pz
An sp hybridised carbon atom
sp hybrid
C2py
sp hybrid
15
Triple bonding in
Ethyne
• Two sp hybridised carbons can form a
covalentbond.
• Other hybrid orbitals covalently bond to two
hydrogens.
C2pz
C2py
C2pz
C2py
16
Pi () bonding
in Ethyne
16
• Less efficient sideways overlap of the pz and py orbitals
gives two C—C pi () bonds .
• These together with the  bond form the triple bond.
• Two sets of clouds (shown in green and blue) form y
and z bonding orbital.
y
C2pz
C2pz
CH  bond
CH  bond
CC  bond
C2py
C2py
H
C
C
H
z
Ethyne (acetylene)
• Because each carbon is sp hybridised (hybrid
orbitals 180° apart) , ethyne is a linear molecule.
• Pi bonds form a barrel of electron density around the
CC bond.
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16
Bond length—strength
CC bonds
Summary:
• Bond length decreases
from single to double to
triple bond.
• Bond strength increases
from single to double to
triple bond.
pm
17
Functional Groups
Alcohols
18
H
H
H
H
C
H
O
H3C
O
CH3OH
Methanol
Functional Groups
Alcohols
R
..
OH
..
CH3CH2OH
Ethanol
R = alkyl group,
OH = hydroxyl group
18
Functional Groups
Alcohols
Classification:
One R group
R
CH2
OH
R
CH
OH
Primary ( 1 °)
Secondary ( 2 °)
Two R groups
R
R
Three R groups
R
C
R
OH
Tert iary ( 3 °)
18
Functional Groups
Amines
19
• In methylamine, sp3 nitrogen is covalently bonded
to methyl and two hydrogens
N
H
H
C
N
H
CH3
H
H
Methylamine (Methanamine)
H
H
Functional Groups
Amines
• Classified on number of alkyl groups attached to
nitrogen
R NH2
1 hydrogen replaced
P rimary (1°) amine
2 hydrogens replaced
Secondary (2°) amine
3 hydrogens replaced
T ert iary(3°) amine
H
N
R
R'
R''
N
R
R'
19
Functional Groups
Ketones and Aldehydes
R
CHO
R = organic group,
CHO = aldehyde group
R
R
CO
R = organic groups,
CO = ketonic group
O
O
C
R
20
C
H
R
R
Functional Groups
Ketones and Aldehydes
20
Formation of a bond using an sp2 hybrid orbital and
a  bond using the pz enables oxygen to form double
bonds to carbon:
O2pz
C2pz
H
H
:
CO -bond
Polarised -molecular
orbital
:
H
120°
H
C
O
Functional Groups
Ketones and Aldehydes
• Carbon is positively polarised and oxygen
negatively polarised
• Carbonyls are best seen as:

C
+
–

O
20
Functional Groups
Carboxylic acids
R
R = alkyl group,
CO2H = carboxyl group
CO2H
O
C
OH
21
Functional Groups
Carboxylic acids
21
• Why acidic?
• In water they ionize partially
H
O
O
R C
O
H
O
Ka
R C
H
O
+ H3 O (Hydroni um ion)
Carboxylate ani on
[RCO2-][H3O+]
K a = [RCO H]
2
K
p a = -log K a
Functional Groups
Carboxylic acids
21
• Resonance:
• Negative charge is on both oxygens
O
O
R C
R C
O
O
R
C
O
O
Resonance hybrid
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