Organic Chemistry I - Reactions of Alkanes - Quick Guides | Chemistry

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Study GuideOrganic Chemistry I–Reactions of Alkanes1.Alkanes: HalogenationAlkanes can react withhalogens(such as chlorine or bromine) whenultraviolet (UV) light or heatispresent. This reaction produces ahaloalkane(also called analkyl halide).A common example is thechlorination of methane.When Does the Reaction Occur?Experiments show that:•No reaction occursif the alkane and halogen are kept in the dark and at low temperature•UV light or heat is requiredto start the reaction•Once the reaction begins, thelight or heat source can be removed, and the reaction willcontinue on its ownThis behavior is explained by thereaction mechanism.Why UV Light Is NeededIn methane:•Carbon–hydrogen (C–H) bonds arelow-polarity covalent bonds•The chlorine molecule (Cl₂) also has anonpolar covalent bondUV light has enough energy to:•Break the chlorine–chlorine bond(≈ 58 kcal/mol)

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Study GuideBut it doesnothave enough energy to:•Break the strongercarbon–hydrogen bond(≈ 104 kcal/mol)1.1Formation of Free RadicalsWhen UV light hits a chlorine molecule:•The Cl–Cl bond splits evenly•Each chlorine atom takesone electron•Twochlorine free radicalsare formedAfree radicalis:An atom or group that containsone unshared (unpaired) electronThese free radicals arehighly reactive.1.2Homolytic Bond Cleavage (Initiation Step)The chlorine–chlorine bond breaks byhomolytic fission:•Each atom gets one electron•No ions are formedThis first step is called theinitiation step, because it starts the chain reaction.Once chlorine radicals form, the energy released in later steps keeps the reaction going—evenwithout UV light.Propagation Steps (Chain-Carrying Steps)Propagation Step 1

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Study Guide•A chlorine free radical attacks a methane molecule•A C–H bond breaks homolytically•Products formed:oHydrogen chloride (HCl)oMethyl free radical (·CH₃)This step forms:•One stable product•One new free radical that keeps the reaction goingPropagation Step 2•A methyl free radical reacts with a chlorine molecule•Products formed:oChloromethane (CH₃Cl)oAnew chlorine free radicalThis chlorine radical can now repeat the cycle.These propagation steps explain why the reaction continues once it starts.

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Study Guide1.3Termination Steps (Chain-Stopping Steps)Atermination stepoccurs whentwo free radicals combineto form a stable molecule.Common termination reactions include:1.A methyl radical + a chlorine radical → chloromethane2.Two methyl radicals → ethane3.Two chlorine radicals → chlorine molecule

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Study GuideTermination stepsremove free radicals, which slows and eventually stops the reaction.Overall Nature of the ReactionThis reaction is called afree-radical chain reactionbecause:•It begins withfew free radicals•Propagation stepsincrease the number of radicals•Toward the end, radicals collide with each other more often•Termination steps become dominantAll of these steps occurextremely fast, and product formation typically happens inmicroseconds.Key Takeaways•Alkanes react with halogens only in the presence ofUV light or heat•The reaction formshaloalkanes•UV light breaks the halogen bond to formfree radicals•The reaction proceeds by:oInitiationoPropagationoTermination•This is a classicfree-radical chain reaction2.Alkanes: Kinetics and RateNot all chemical reactions happen easily or quickly.Most reactions need energybefore they canbegin. This section explainswhy energy is needed,what controls reaction speed, andhowtemperature affects reaction rate.

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