Organic Chemistry II - Alkyl Halides | Chemistry

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Study GuideOrganic Chemistry II–Alkyl Halides1.Nucleophilic Substitution ReactionsAlkyl halides are very reactive compounds in organic chemistry. One of their most important reactionsisnucleophilic substitution, where one atom or group is replaced by another.In this reaction, anucleophileattacks the carbon atom bonded to a halogen andreplaces thehalogen atom. The halogen then leaves the molecule as ahalide ion.1.The General ReactionA nucleophilic substitution reaction can be written in a simple form:Nu⁻+ R–X → R–Nu + X⁻Where:•Nu⁻= nucleophile•R–X= alkyl halide•R–Nu= product•X⁻= halide ionThe halogen atom that leaves the molecule is called theleaving group.2.Common NucleophilesSeveral negatively charged or electron-rich species commonly act as nucleophiles. Some importantexamples include:•Hydroxide ion (–OH)•Alkoxide ion (RO⁻)•Cyanide ion (⁻C≡N)•Ammonia or amines (–NH₂and related groups)

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Study GuideThese nucleophiles donate a pair of electrons to form a new bond with carbon.3.Typical Nucleophilic Substitution ReactionsHere are some common reactions involving alkyl halides:1. Formation of Alcohols•Hydroxide ion + alkyl halide → alcohol + halide ion•The–OH group replaces the halogen atom.2. Formation of Ethers•Alkoxide ion + alkyl halide → ether + halide ion•The reaction forms an oxygen-containing ether.3. Formation of Nitriles•Cyanide ion + alkyl halide → nitrile + halide ion•The–C≡N group becomes part of the carbon chain.4. Formation of Amines•Primary amine + alkyl halide → secondary amine + halide ion•The nitrogen-containing group replaces the halogen.

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Study GuideIn all these reactions, thehalogen leaves as a halide ion.4.Leaving Groups and Reaction Name•The halogen ion that departs from the molecule is known as theleaving group.•Because a nucleophile replaces another group, the overall process is called anucleophilicsubstitution reaction.Key Takeaway•Alkyl halides commonly undergonucleophilic substitution reactions.•Anucleophilereplaces a halogen atom attached to carbon.•The halogen leaves as ahalide ion, called theleaving group.•Common nucleophiles include–OH, RO⁻,⁻C≡N, and amines.•These reactions form important functional groups such asalcohols, ethers, nitriles, andamines.•The general reaction is:Nu⁻+ R–X → R–Nu + X⁻.2.Leaving GroupFor a molecule to take part in anucleophilic substitution reaction, it must meet two importantconditions:1.It must contain apolar bond, and2.It must have agood leaving group.Both of these features make it easier for the reaction to occur.1.What Is a Leaving Group?Aleaving groupis an atom or group that candetach from the moleculeduring a reaction and existon its own afterward.For a group to be a good leaving group, it must:

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Study Guide•Be able to exist independently as astable species•Form aweakly basic ion or molecule•Be comfortable carrying anegative chargeThe more stable the leaving group is after it leaves, the easier the reaction will be.2.Why Stability MattersWhen a leaving group departs, it usually takes the bonding electrons with it and becomesnegativelycharged. Good leaving groups can stabilize this negative charge in one of two ways:•High electronegativityElectronegative atoms hold negative charge well.•Delocalization of chargeSpreading the charge over multiple atoms increases stability.Groups that can do this easily are much better leaving groups.3.Why Halogens Are Good Leaving GroupsHalogen atoms (such asCl, Br, and I) make excellent leaving groups because:•They arehighly electronegative•They formstable halide ions•They areweak bases, which makes them less likely to react againBecause of these properties, halogens commonly serve as leaving groups in nucleophilic substitutionreactions.Key Takeaway•A nucleophilic substitution reaction requires apolar bondand agood leaving group.•A good leaving group must be able toexist independentlyafter leaving.•Good leaving groups formstable, weakly basic ions or molecules.

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Study Guide•Stability comes fromhigh electronegativityorcharge delocalization.•Halogensare good leaving groups because they formstable halide ions.•Better leaving groups make substitution reactionsfaster and easier.3.Nucleophilic Substitution Reactions: MechanismsExperimental studies of nucleophilic substitution reactions—especially those involvingopticallyactive compounds(molecules that rotate plane-polarized light)—show that these reactions canoccur bytwo distinct mechanisms:•SN2 mechanism•SN1 mechanismEach mechanism differs in how the reaction occurs, how fast it proceeds, and how molecular structureaffects the outcome.1.Overview of SN2 and SN1 MechanismsSN2 Mechanism•Followssecond-order kinetics•Reaction rate depends ontwo reactants: the substrate and the nucleophile•The transition state containsboth species•The term SN2 meanssubstitution nucleophilic bimolecularSN1 Mechanism•Followsfirst-order kinetics•Reaction rate depends ononly one reactant: the substrate•The key intermediate containsonly the substrate•The term SN1 meanssubstitution nucleophilic unimolecular

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Study Guide2.The SN2 Mechanism: One Step, One MotionIn an SN2 reaction, the alkyl halide has apolar carbon–halogen bond. The mechanism occurs in asingle, concerted step.Figure 1The SN2 mechanism can also be illustrated as shown in Figure 2.Figure 2How It Happens•The nucleophile attacks the carbon atom from theback side, opposite the leaving group•This attack occurs on theantibonding (back) lobeof the carbon orbital•A short-livedactivated complex (transition state)forms, where the carbon is temporarilybonded to both the nucleophile and the leaving group•As the leaving group departs, the new bond forms simultaneously

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Study Guide3.Geometry and Inversion of Configuration•In the transition state, the carbon atom briefly adopts atrigonal bipyramidal shape•After the leaving group exits, the carbon returns to a tetrahedral shape•However, the spatial arrangement of groups around carbon isinvertedThis inversion is called theWalden inversionand is a defining feature of SN2 reactions.Figure 3Why Backside Attack Is RequiredThe nucleophile must attack from the sideopposite the leaving groupbecause:•The front side is blocked by the leaving group•The antibonding orbital is accessible only from the backAs a result,front-side attack does not occurin SN2 reactions.4.Steric Hindrance in SN2 ReactionsSN2 reactions are very sensitive tosteric hindrance.•If many or bulky groups surround the reacting carbon, the nucleophile has difficultyapproaching•Increased crowding slows the reaction or stops it entirely

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Study GuideReactivity Trend•Methyl (CH₃–X): fastest•Primary alkyl halides: fast•Secondary alkyl halides: slow•Tertiary alkyl halides: no reactionBulky groups create greater steric hindrance and reduce the reaction rate.5.Solvent Effects on SN2 ReactionsProtic Solvents•Can form hydrogen bonds•Strongly solvate nucleophiles•Lower nucleophile reactivity•Decrease SN2 reaction ratePolar Aprotic Solvents•Do not hydrogen-bond to nucleophiles•Solvate only the accompanying cation

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Study Guide•Increase nucleophile reactivity•Increase SN2 reaction rateLower activation energy leads to faster reactions.Figure 4 illustrates the effect of solvent polarity on the energy of activation and, thus, the rateof reaction.Figure 46.The SN1 Mechanism: Two Steps, One IntermediateThe SN1 mechanism occurs intwo separate steps.Step 1: Formation of the Carbocation (Slow Step)•The alkyl halide breaks apart•Acarbocationand aleaving group anionare formed•This step determines the reaction rateStep 2: Nucleophilic Attack (Fast Step)•The nucleophile attacks the carbocation•A new bond forms to give the substitution product

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Study GuideBecause only the substrate is involved in the slow step, the reaction isunimolecular.7.Carbocations and Optical Activity•Carbocations aresp²-hybridizedandplanar•The nucleophile can attack fromeither sideof the planeAs a result:•An optically active starting material produces aracemic mixture•Equal amounts of both enantiomers are formedThis loss of stereochemical information is a hallmark of SN1 reactions.Figure 5Key Takeaway•Nucleophilic substitution occurs bySN2 or SN1 mechanisms•SN2 reactionsare:

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