Organic Chemistry Lesson 4 Alkanes

Building Organic Compounds Use molecular models to make any of the compounds mentioned in your mind map: Draw it (structural and skeletal) Name it Give it to a friend and challenge them to do the same Only go up to 10 carbons Only include branched chains for the alkanes

Compounds can often have common or popular names Methyl 2-hydroxybenzoate is better known as oil of wintergreen Acetone = propanone, CH3-CO-CH3 Isopropyl alcohol = propan-2-ol, CH₃CH(OH)CH₃ Formaldehyde = methanal, CH3CHO Alcohol = ethanol, C2H5OH Acetic acid = ethanoic acid, CH3COOH Ether = diethyl ether, C2H5-O-C2H5 White, granulated sugar = sucrose = α-D- glucopyranosyl-(1,2)- β-D-fructofuranoside, C12H22O11

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Lesson 4: Alkanes Objectives: Explain the stability of the alkanes Observe the combustion of alkanes Describe the free-radical substitution reactions of alkanes and its mechanism Observe the free-radical substitution of hexane

Trends in physical properties of alkanes Alkanes are non-polar Insoluble in water (oil and water don’t mix) Nearly all alkanes have densities less than 1.0 g/mL and are therefore less dense than water (oil and grease float on top of water). Methane ethane propane butane pentane hexane heptane octane nonane decane Volatility Branched chain alkanes straight chain alkanes 2-methylpropane (-0.11C) boiling point butane (-0.5 C)

Combustion of alkanes Alkanes really don’t do much Combustion is of one of two notable reactions (this is why we use them for fuels) Complete combustion: alkane + oxygen  carbon dioxide + water Incomplete combustion: Alkane + oxygen  carbon + carbon monoxide + carbon dioxide + water The amounts of C, CO and CO2 will vary depending on conditions Incomplete combustion depends on the availability of oxygen Task: Observe the combustion of the gas from the gas taps (propane/butane mix) and of a small amount hexane (in spirit burners). Hold the end of a clean boiling tube just over the flame for 15 seconds, this will collect soot (black carbon) from the flame. Record all observations clearly and try to account for them Include balanced equations to describe the (complete) combustion

Why Good Fuels? To do: Use Table 12 in the data booklet to help you determine the trend in energy released per gram by combustion of the alkanes. Use bond enthalpies to help you explain the trend noted above. What do you think should be the characteristics of a good fuel? Use the above to decide and explain which out of methane and octane is a better fuel.

Why so boring stable? There are at least two reasons why alkanes are so unreactive Task: Think back to your knowledge of molecular structure, and look at the tables of bond enthalpies in the data booklet to see if you can work out why.

More organic vocabulary Alkanes are saturated hydrocarbons. They contain all single carbon-carbon bonds. [Unsaturated hydrocarbons contain double or triple carbon-carbon bonds.] Alkanes are also aliphatic, meaning the carbon atoms form open chains. [In contrast to aromatic compounds which contain benzene rings.] Alkanes contain only strong C-C (348 kJ/mol) and C-H (412 kJ/mol) bonds. Alkanes are non-polar. They have weak intermolecular forces and they are not susceptible to attack by common reactants.

Halogenation Alkanes will undergo halogenation if reacted with a halide in the presence of u.v. light. For example: C2H6(g) + Cl2(g) CH3CH2Cl(g) + HCl(g) ethane chloroethane This reaction is an example of free radical substitution u.v.

Radicals Radicals are species with unpaired electrons They are crazy reactive Halogens form radicals when hit by uv light of the right frequency: Cl2 2 Cl• The dot after the Cl represents the unpaired electron and tells us we have a radical This process is called homolytic fission – the bond breaks equally with one electron going to each chlorine Task: draw Lewis structures for the Cl2 molecule and each of the Cl• radicals. Show the process using single-sided curly arrows, sometimes called fishhooks, to show the movement of a single electron. u.v. What is the difference between a free radical and an ion? A free radical has an unpaired electron but no net charge. An ion carries a charge.

Reaction Mechanism: Free Radical Substitution Cl2 2 Cl• Cl• + C2H6  C2H5• + HCl C2H5• + Cl2  C2H5Cl + Cl• C2H5Cl + Cl•  C2H4Cl• + HCl C2H4Cl• + Cl2  C2H3Cl2 + Cl• Cl• + Cl•  Cl2 Cl• + C2H5•  C2H5Cl C2H5• + C2H5•  C4H10 Initiation Radicals formed by homolytic fission Propagation These steps feed each other the radicals needed to continue. This type of reaction is called a chain reaction. Termination Any two radicals can combine to terminate the reaction Concentration of radicals is low so this is a rare event u.v. A single radical can cause thousands of cycles of the propagation stage before it reaches termination This same mechanism applies to all of the halogens The alkane can be substituted multiple times, until every H has been replaced

Try it Yourself Place approx 1 cm3 of hexane into two test tubes. Add roughly 1 cm3 of bromine water to each and stopper them, then give them a good shake. Leave one test tube in the classroom but take the other outside and shake it in direct sunlight. Record and explain all observations. Write equations showing the mechanism of the reaction. Draw full structural and skeletal formulas of at least 6 possible products, and name each one. Bromine water becomes decolourized as the photochemical reactions occurs in UV light. When no reaction occurs then there is no change in colour of the bromine water.

Extension: Research the role of free radical reactions in the depletion of the ozone layer or the combustion of hydrocarbon alkanes on global warming and climate change. Remember CO2 and H2O are greenhouse gases. CO is toxic to human. Unburned carbon is particulate matter.

Key Points Alkanes are fairly unreactive They release a lot of energy on combustion, and are easy to handle, store and transport which makes them good fuels Alkanes undergo free radical substitution to form halogenoalkanes and a hydrogen halide in the presence of UV light