Organic Functional Groups: Halocarbons
Organic Functional Groups: Halocarbons
Representing Covalent BondsThe 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 UsedBonding pairs are shown as dashes;
lone pairs are shown as dots.
Non-Polar Covalen BondsIn 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 BondsIn 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 PolarityCarbon 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
BondingThe 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!
MethaneBoiling Point: -160CChloromethaneBoiling Point: -24CAll
non-polar bondsC-Cl bond is polarFunctional GroupsWhen 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 17So 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 HalogensThe 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 elementsHalocarbons 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 HalocarbonsNaming 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
FollowingCH3CHFCH3CCl2HCH2CH2CH2ClCCl2F2CH3BrCH3CH2CHICH3Cl2C=CCl2CCl4Draw
the Following Halocarbons2-Chloro pentane1,1,1 trichloro
ethane3-fluoro propene2,4 dibromo hexane1,2 diiodo butaneHalocarbon
ReactionsSaturated HydrocarbonsHalogens will replace a hydrogen
atom:
CH3CH3(g) + Cl2(g) CH3CH2Cl (l) + HCl(g)
This is called a substitution reaction.Halocarbon
ReactionsUnsaturated HydrocarbonsLike 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?CH4C2H4C3H4C4H8C5H12C6H10C6H14
Halocarbon UsesHalocarbons are not naturally occurringTheir 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 HalocarbonsCFCs 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
Organochlorine pesticides compounds such as DDT, which
concentrate up the food chain, are still used in the developing
world.
