Transcript Halocarbonsx

Organic Functional Groups:
Halocarbons
Representing Covalent Bonds
• The valence electrons of elements are often
represented using dot diagrams
Dot diagrams can be used to show bonding pairs of electrons
for elements and compounds.
Hybrid Diagrams Are Also Used
• Bonding pairs are shown as dashes; “lone pairs”
are shown as dots.
Non-Polar Covalen Bonds
• In elements that form covalent bonds,
electrons are shared equally between atoms.
• These bonds are called non-polar, because
the electrical charge is evenly distributed
Polar Covalent Bonds
• In many compounds, atoms of different
elements share their electrons unequally –
one element gets them a bigger share of the
time than the other one.
• The chlorine atom is more electronegative,
that is, it attracts the electron more than the
hydrogen does.
Consequences of Bond Polarity
• Carbon and hydrogen share their electrons
equally – they form non-polar covalent bonds.
• Compounds with non-polar bonds tend to
have weaker attractive forces, lower melting
points and boiling points.
• Small non-polar molecules tend to be gases,
larger ones liquids at room temperature.
Polar Covalent Bonding
• The halogens are more electronegative than
carbon, making for more polar covalent bonds.
• Halocarbon compounds have small dipoles,
partially positive and negative ends, which
change the physical and chemical properties of
compounds significantly.
• Halocarbons have greater intermolecular forces,
higher melting and boiling temperatures, and are
often liquids at room temperature.
One Atom Really Makes a Difference!
C-Cl bond is polar
All non-polar
bonds
Methane
Boiling Point: -160°C
Chloromethane
Boiling Point: -24°C
Functional Groups
• When a highly electronegative element (one that
attracts electrons) is part of an organic compound,
they are often called functional groups.
• Functional groups have a profound affect on the
physical and chemical properties of atoms.
• We shall look at three classes of organic compounds
with polar functional groups: halocarbons, oxygenated
compounds such as alcohols, ethers, carbonyl
compounds, and esters, and nitrogenous compounds
like amines and amides.
The
Halogens
The Halogens:
Periodic Table, Group 17
• So far we have considered organic compounds
containing only two elements.
• Now we will expand our horizons further. First we
will consider a group of elements collectively
known as the halogens.
• On the periodic table, elements in columns are
known as groups or families.
• Elements in the same group have similar chemical
and physical properties, and make similar kinds of
compounds.
Group 17: The Halogens
• The halogens are in the second column from the
right.
• Astatine (At) is not included: it is unstable and
extremely rare.
• Halogens each have seven valence electrons – three
more than carbon.
• Halogens are very hungry to gain an 8th electron.
When they do, the eight electrons collectively drop
in energy and are more stable.
• The halogens tend to share electrons, but not
equally. They tend to make polar covalent bonds
with other elements on the right side of the periodic
table.
Halogens form diatomic elements
• Halocarbons do not generally exist in nature.
Because of their reactivity, halogens are
always found as negative ions, which do not
react with hydrocarbons.
• Halocarbons are made in the laboratory using
the halogen elements in their pure form,
which is a diatomic molecule:
F2 Cl2 Br2 I2
For fluorine:
Naming Halocarbons
• Naming a halocarbon is similar to naming a
hydrocarbon.
• You need to specify the name of the halogen,
the number of halogens (if greater than one),
and their positions on the hydrocarbon.
• Halogens written as prefixes such as chloro-,
fluoro-, bromo-, etc.
Name the Following
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CH3CHFCH3
CCl2HCH2CH2CH2Cl
CCl2F2
CH3Br
CH3CH2CHICH3
Cl2C=CCl2
CCl4
Draw the Following Halocarbons
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2-Chloro pentane
1,1,1 trichloro ethane
3-fluoro propene
2,4 dibromo hexane
1,2 diiodo butane
Halocarbon Reactions
• Saturated Hydrocarbons
Halogens will replace a hydrogen atom:
CH3CH3(g) + Cl2(g) CH3CH2Cl (l) + HCl(g)
This is called a substitution reaction.
Halocarbon Reactions
• Unsaturated Hydrocarbons
Like hydrogen, halogens break the double
bond and add to the molecule in two
places:
H2C=CH2 + F2  CH2FCH2F name the product!
This is called an addition reaction.
Substitution or Addition?
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CH4
C2H4
C3H4
C4H8
C5H12
C6H10
C6H14
Halocarbon Uses
• Halocarbons are not naturally occurring
• Their most important use is to build large
organic molecules – their ability to substitute
for hydrogen is matched by their ability to be
removed!
• Many industrial uses of halocarbons have
been limited because of their toxicity, but they
are still widely used.
Classes of Halocarbons
• CFCs – chloroflurocarbons – formerly used in
air conditioners, hairspray, and refrigerators
• HFCs – hydrofluorcarbons – currently used as
a replacement for CFCs, these are major
greenhouse gases
• PCBs – polychlorinated biphenyls – used in
heating and cooling systems; they degrade
into dioxins, which very toxic substances
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• Organochlorine pesticides – compounds such
as DDT, which concentrate up the food chain,
are still used in the developing world.