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 Covalent Bonds In elements that form covalent bonds, electrons are shared equally between atoms. These bonds are 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 the halogens.

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 CH3CHFCH3 CCl2HCH2CH2CH2Cl CCl2F2 CH3Br CH3CH2CHICH3 Cl2C=CCl2 CCl4

Draw the Following Halocarbons 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? 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.

Halocarbons you live with everyday Stain resistant coatings on furniture No stick surfaces on pots and pans Wrinkle-free fabrics Plastics, like PVC Greasy food packages coated with PFCs

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  Organochlorine pesticides – compounds such as DDT, which concentrate up the food chain, are still used in the developing world.