Download II.I Corhon compounds lI.2 Hydrocorhons

| 1.2 Hydrocarbons
II.I Corhoncompounds
,17
)
AIM: To describethe woys thot corbon otomsare bondedin
organic compounds.
Focus
Carbon atoms combine with
other carbon atoms and with
atoms of other elements.
organic compounds exhibit dramatically different chemicar and physical
properties because they have different structures. At standard conditions,
some are solids,some are liquids, and some are gases.Sometaste sweet,and
others taste sour. some are poisons, but others are essentialfor life. In order
to understand these properties of organic molecules, it is necessaryto
explore and understand their structures.Three simple principles can provide
a basic understanding of the structure and chemistry of organic molecules:
l. Carbon atoms can form covalent bonds with hydrogen atoms.
2. carbon atoms can form covalent bonds with other carbon atoms to
build carbon chains.
3. carbon atoms can form covalent bonds with other elements, especially
oxygen,nitrogen, phosphorus, sulfur, and the halogens.
We begin our study of organic chemistrywith bonding between carbon
and hydrogen atoms, and then we will examine bonding between carbon
atoms.we will discusscarbon'sbonding to other elements in later chapters.
lI.2 Hydrocorhons
AIMS: Todrow the electrondot ond a structural formulo of
methone.To useorbitol hybridizotionto describethe
shopeof the methonemolecule.
Methane is the simplest organic
molecule.
Hydrocarborrs are compounds whose molecular structures c:ontainonly
hydrogen and carbon The simplest hydrocarbons are the alkanes-hydro
carbons that contain only single coualentbonds.The smallest organic molecule is the alkane called methane (cHn;. Methane, a gas at standard temperature and pressure, is the major component of natural gas. It is
sometimescalled"marsh gas"becauseit is formed bythe action of bacteria
on decaying vegetation in swamps and other marshy areas.
The carbon-hydrogen
bond in methane
A carbon atom contains four valence electrons. Four hydrogen atoms, each
with one valence electron, can form four covalent carbon-hydrogen bonds.
This combination is a molecule of methane:
H
.C.
+
4H.
H:C:H
ii
Carbon
atom
Hydrogen
atoms
Methane
molecule
There is an important principle here: Becausea carbon atom containsfour
ualenceelectrons,in making organic compounds it nearly alwaysforms four
518
tl CarbonChainsand Rings
CHAPTER
Anaerobic (in the absenceof
oxygen)digestion of sewage
sludgeproducesmethane gas.
It is estimatedthat each ton of
solid waste can be digestedto
produce 85 m3 of methane.
\
coualentbonds.Remembering this principle will help you to writefcomplete
and correct structures for organic molecules.
As you can see,the molecular formula of methane is CHa.However,molecular formulas are not very useful in organic chemistry.Molecular formulas
cannot, by themselves,contain enough information about a molecule, such
as the €rrangement of the atoms and the bonding between the atoms. For
this reason,organic chemists prefer to write structural formulas-formul.as
that show coualent bonds and the arrangenxentof atoms in each molecule.
Organic chemists usually abbreviate the bonding electron pairs in carboncontaining molecules as short lines. It will help to remember that the line
between the atomic symbols representstwo electrons.For example:
H
| ..----:
H - C - i H ,' L i t t e r e P r e ' e n t t pair
sharedelertron
T
H
Methane
molecule
The shape of methane
| 1.1
Figure
modelof the
Ball-and-stick
methane
molecule.
You may recall from Section 5.10 that electron-pair repulsion theory predicted a tetrahedral shapefor the methane molecule. This prediction agrees
with experimental work that shows that methane is a completely sl.rnmetrical tetrahedral molecule; all the C-H bonds are identical, and all the
H-C-H
bond anglesare 109.5degrees(Fig.11.1).
As we discussedin Section3.5,electronconfigurationsin atoms can be
describedby atomic orbitals.The bonding in methane and other molecules
also can be describedby atomic orbitals.\Mhenone atomic orbital overlaps
with another atomic orbital, each with one electron, the pair of electrons is
shared, and a covalent bond is formed. We can illustrate this concept with
the hydrogen molecule, H2.The ls atomic orbitals of two hydrogen atoms
overlap in the formation of a hydrogen molecule. TWoelectrons, one from
each hydrogen atom, are available for sharing and a covalent bond is
formed (Fig.11.2).
However, if we attempt to generate a picture of methane by simple
atomic orbital overlap, the method fails. A carbon atom has only two
unpaired electrons,one in each of two 2p orbitals:
ru runnI
ls
2s
2p
If the two electrons of the 2p orbitals were each shared with an electron of
the ls orbital of hydrogen atoms, we would get a molecule with the formula
F i g u r e1 1 . 2
The overlapof the ls orbitalsof
two hydrogenatoms,eachwith
one electron,producesa covalent
electronpair bond.The productis a
hydrogenmolecule,H2.
s atomlc
orbital
s atomic
orbital
Sigma-bonding
orbital
I 1.2 Hydrocarbons 519
CH, instead of CH4. One solution to this dilemma iI orbital
hybridization-the
mixing of two or rnore dffirent atonxic orbitals. rn
methane, a c.ubon atom's 2s orbital and three 2p orbitals mix to form four
identical sp3 (read "s-p-three") hybrid atomic orbitals, each with one electron. Note that the number of hybrid orbitals that are formed is equal to the
number of orbitals that are mixed. Becauseeach sp3hybrid orbital is composed of 75Toof dumbbell-shaped p orbitals and only 2SToof a spherical s
orbital, its appearance is similar to a fattened p atomic orbital. Figure 11.3
shows how the balloon-shaped sp3 orbitals point toward the corners of a
Beginwith 2s and 2p atomic orbitals of carbon.
||
I
I
Hybridize the 2^sand the 2p atomic orbitals
to form four sl hvbrid atomic orbitals
t
Overlap each sf hybrid atomic orbital
with a lshydrogen atomic orbital
to form four equivalent sigma-bonding orbitals
I
FigureI 1.5
Stepsleadingto an orbitaldescriptionof the methanemolecule.
Hybridization
of the 2s andZp
orbitalsof a carbonatomproduces
foursp3hybridatomicorbitalsthat
pointto the cornersof a tetrahedron.Eachof thesehybridorbitals
overlaps
with a ls orbitalof hydrogento producea methanemolecule.
ls
@'.. :
\
Methane
,20
CHAPTER
I I Carbon
Chains
and Rings
\
tetrahedron. The tetrahedral angle of 109.5degreesbetween u.tyftto hybrid
orbitals minimizes unfavorable interactions among the orbitals. Now the
four sp3orbitals of carbon, each of which has one electron, can overlap with
the 1sorbitals of four hydrogen atoms, each with one electron. The product
is methane, CHa.The carbon-hydrogen bonds in methane are all of a type
called sigma bonds.Imagine slowly rotating the bond between a carbon
nucleus and a hydrogen nucleus. The bond looks exactly the same throughout the rotation because it is qrmmetrical along the bond axis. A coualent
bond that is completely symmetrical along the axis connecting two atomic
nuclei ls a sigma bond (the Greek letter sigma is o). The covalent bond in
the hydrogen molecule is a sigma bond. Carbon-hydrogen single bonds,
carbon-carbon single bonds, and single bonds between carbon and other
elements are also sigma bonds.
The structure of the resulting methane molecule is in agreement with
experiment; it has four identical C-H bonds and is tetrahedral. Since sp"
orbitals extend farther in space than either s or p orbitals, the overlap of a
carbon sp3orbital with a hydrogen 1sorbital is greater than is possible with
either aZs or a2p orbital of carbon. This greater overlap results in an unusually strong covalent bond.
bonds
I l,I Corbon-carbon
AIM: To showwith on electrondot ond o structurolformula how
o corhon-corbonbond is formed in ethone,CzH*
Ethane is the smallest alkane
containing a carbon-carbon
bond.
The unique ability of carbon to make stable carbon-carbon bonds and form
chains is the major reason for the vast number of organic molecules.A few
other elements, notably silicon, form short chains, but most silicon chains
are unstable in an oxygen environment.
Like methane, ethane is a gasat standard temperature and pressure.Let
us seehow ethane could be formed from carbon and hydrogen. TWocarbon
atoms could share a pair of electrons to form a carbon-carbon covalent
bond:
Carbon
atoms
Electron
pair bond
Now the remainder of the valence shell electrons of each carbon could be
paired with the electrons from six hydrogen atoms to make a molecule of
ethane:
HH
C-C.
+ 6H. _-
H-C-C-H
HH