Chapter A3 A3.2 – Alkenes & Alkynes
Alkenes & Alkynes • Hydrocarbons with double and triple bonds are important in the petrochemical industry. • they are the starting materials for themanufacture of important products such as plastics.
Alkenes & Alkynes • A double or triple bond affects the chemical properties of the molecule • If an organic compound has carbon-carbon double bonds, it is said to be unsaturated. • these compounds tend to react readily with small diatomic molecules such as Br2 and H2. • this reaction is called an addition reaction.
Alkenes & Alkynes • The addition of enough hydrogen molecules is called hydrogenation & results in a saturated hydrocarbon. • The hydrogen atoms take the place of the double bond.
Alkenes & Alkynes • Hydrocarbons with double bonds are called alkenes, & with triple bonds are called alkynes. • These compounds are named in a similar way to alkanes, but their names will end in –ene & –yne. • The general formula for alkenes is CnH2n & for alkynes is CnH2n-2.
Alkenes & Alkynes Name Formula ethene C2H4(g) propene C3H6(g) butene C4H8(g) pentene C5H10(l) hexene C6H12(l) heptene C7H14(l) octene C8H16(l) nonene C9H18(l) decene C10H20(l) - ene CnH2n Name Formula ethyne C2H2(g) propyne C3H4(g) butyne C4H6(g) pentyne C5H8(l) hexyne C6H10(l) heptyne C7H12(l) octyne C8H14(l) nonyne C9H16(l) decyne C10H18(l) - yne CnH2n-2
Naming Alkenes & Alkynes • Since the location of a multiple bond affects the chemical & physical properties of a compound, the name of the compound must specify the location of the multiple bond.
Naming Alkenes & Alkynes • E.g. pent-2-ene indicates the double bond is located between the 2nd and 3rd carbons of the parent chain.
Naming Alkenes & Alkynes The rules for naming are similar to alkanes, with three additional rules: • The parent chain mustcontain the multiple bond. • The parent chain is numbered from the end closest to the multiple bond (not closest to the first branch as for alkanes).
Naming Alkenes & Alkynes 3. The end of the name of the parent chain is preceded by the number of the carbon on which the multiple bond begins.
Practice Problem Name the following compound.
Practice Problem (Solution) • Just like before, begin by identifying the parent chain, but this time number it beginning at the end closest to the multiple bond. • Note the location of the multiple bond, and the location and size of any branches.
Practice Problem (Solution) Remember that the location of the multiple bond takes precedence over the location of any branches.
Isomers • Isomers are compounds containing the same number of carbons & hydrogens, but different arrangements of them. • Isomers have the same formula, but different structural diagrams.
Practice Problem: • Draw the following isomers • pentane • 2-methylbutane • 2,2-dimethylpropane • What is the chemical formula for each isomer? • Why is 2-ethylpropane not an isomer of the three compounds listed above?
Fats & Oils • Animal fats and plant oils are a source of energy, but also provide food with taste & provide a feeling of being full. • Fatty acids are organic molecules made of • a long chain of carbons • a -COOH group at one end • a methyl group at the other end
Oils • Structure of oils cause the molecule to have a bend in it, & typically don’t pack together as tightly. They tend to be liquids at room temperature.
Fats • Structure of fats make the molecule straight, so they can pack together & tend to be solids, which makes them harder for your body to break down.
Essential Fatty Acids • Essential fatty acids are fats your body can’t make, & are found only in certain foods. • “Omega-6” fatty acids are found in margarine, processed foods, corn oil & soybean oil. • While these fats are important for clotting & inflammation, many people eat too many omega-6s.
Essential Fatty Acids “Omega-3” fatty acids are found in some fish, flax seeds, & enriched eggs. • These fats have the opposite effect as omega-6s – They reduce swelling & slow blood clotting, so a balance between the two is important.
Unsaturated Fats • Monounsaturated fat – fatty acids that have take the place of the double bond. • Polyunsaturated fat – fatty acids with more than one double bond, (and fewer hydrogens). • Unsaturated fats tend to be better for the body.
Trans Fats • “Cis” and “Trans” refer to different 3D arrangements of a molecule. • “Cis” fats are naturally occuring, but can be made into “Trans” fats through a process called hydrogenation.
Trans Fats Trans fats have become popular with food manufacturers & restaurants because • They are in solid form, so they are easier to work with than oils. • They are softer and easier to spread that typical animal fats. • You need less trans fats to have the same effect, so they qualify for a “low fat” label, even though the fats are much worse for you.
Trans Fats Because they are synthetic, trans fats cannot be easily broken down in your body, & are the worst kinds of fat. Trans fats are found in processed foods & fast foods. Because food manufacturers are allowed to put up to 0.5g of trans fats in their food & still label it as “trans fat free”, be sure to check ingredients labels for partially hydrogenated oils
The Stability of Organic Compounds • A more stable compound has a higher melting point and boiling point, is less reactive, and is more likely to be found in solid form. • A compound will be more stable if it is larger • this is because it has more carbon-carbon bonds to break during heating or chemical reaction.
The Stability of Organic Compounds • A compound will also be more stable if it is saturated with hydrogens, that is, if it does not contain multiple bonds. • This is because it has more hydrogen bonds are a stabilizing force in a molecule, whereas a double or triple bond is relatively easy to break
Practice Problem: • Rank the following compounds in order of lowest to highest boiling point • pentane, butane, propane • propene, propane, propyne • Rank the following compounds in order from least to most reactive • ethyne, nonane, pentene • octane, heptene, octene
Solubility • Recall from Chapter A1, polar solutes dissolve easily in polar solvents. • Water is polar, so it easily dissolves ionic compounds, acids, and polar molecular compounds. • E.g. ethanol dissolves in water because it is polar.
Solubility • Alkanes, alkenes and alkynes are non-polar, so would dissolve better in a non-polar solvent (e.g. oil) than a polar one (e.g. water).
Homework • Organic chemistry worksheet Unit A Exam • Date: