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Organic Chemistry II - Alkyl Halides

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    Study Guide Organic Chemistry II – Alkyl Halides 1. Nucleophilic Substitution Reactions Alkyl halides are very reactive compounds in organic chemistry. One of their most important reactions is nucleophilic substitution , where one atom or group is replaced by another. In this reaction, a nucleophile attacks the carbon atom bonded to a halogen and replaces the halogen atom . The halogen then leaves the molecule as a halide ion . 1. The General Reaction A 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 ion The halogen atom that leaves the molecule is called the leaving group . 2. Common Nucleophiles Several negatively charged or electron - rich species commonly act as nucleophiles. Some important examples include: • Hydroxide ion ( – OH) • Alkoxide ion (RO ⁻ ) • Cyanide ion ( ⁻ C≡N) • Ammonia or amines ( – NH ₂ and related groups)

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    Study Guide These nucleophiles donate a pair of electrons to form a new bond with carbon. 3. Typical Nucleophilic Substitution Reactions Here 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 Guide In all these reactions, the halogen leaves as a halide ion . 4. Leaving Groups and Reaction Name • The halogen ion that departs from the molecule is known as the leaving group . • Because a nucleophile replaces another group, the overall process is called a nucleophilic substitution reaction . Key Takeaway • Alkyl halides commonly undergo nucleophilic substitution reactions . • A nucleophile replaces a halogen atom attached to carbon. • The halogen leaves as a halide ion , called the leaving group . • Common nucleophiles include – OH, RO ⁻ , ⁻ C≡N, and amines . • These reactions form important functional groups such as alcohols, ethers, nitriles, and amines . • The general reaction is: Nu ⁻ + R – X → R – Nu + X ⁻ . 2. Leaving Group For a molecule to take part in a nucleophilic substitution reaction , it must meet two important conditions: 1. It must contain a polar bond , and 2. It must have a good leaving group . Both of these features make it easier for the reaction to occur. 1. What Is a Leaving Group? A leaving group is an atom or group that can detach from the molecule during a reaction and exist on 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 a stable species • Form a weakly basic ion or molecule • Be comfortable carrying a negative charge The more stable the leaving group is after it leaves, the easier the reaction will be. 2. Why Stability Matters When a leaving group departs, it usually takes the bonding electrons with it and becomes negatively charged . Good leaving groups can stabilize this negative charge in one of two ways: • High electronegativity Electronegative atoms hold negative charge well. • Delocalization of charge Spreading the charge over multiple atoms increases stability. Groups that can do this easily are much better leaving groups. 3. Why Halogens Are Good Leaving Groups Halogen atoms (such as Cl, Br, and I ) make excellent leaving groups because: • They are highly electronegative • They form stable halide ions • They are weak bases , which makes them less likely to react again Because of these properties, halogens commonly serve as leaving groups in nucleophilic substitution reactions. Key Takeaway • A nucleophilic substitution reaction requires a polar bond and a good leaving group . • A good leaving group must be able to exist independently after leaving. • Good leaving groups form stable, weakly basic ions or molecules .

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    Study Guide • Stability comes from high electronegativity or charge delocalization . • Halogens are good leaving groups because they form stable halide ions . • Better leaving groups make substitution reactions faster and easier . 3. Nucleophilic Substitution Reactions: Mechanisms Experimental studies of nucleophilic substitution reactions — especially those involving optically active compounds (molecules that rotate plane - polarized light) — show that these reactions can occur by two distinct mechanisms : • SN2 mechanism • SN1 mechanism Each mechanism differs in how the reaction occurs, how fast it proceeds, and how molecular structure affects the outcome. 1 . Overview of SN2 and SN1 Mechanisms SN2 Mechanism • Follows second - order kinetics • Reaction rate depends on two reactants : the substrate and the nucleophile • The transition state contains both species • The term SN2 means substitution nucleophilic bimolecular SN1 Mechanism • Follows first - order kinetics • Reaction rate depends on only one reactant : the substrate • The key intermediate contains only the substrate • The term SN1 means substitution nucleophilic unimolecular

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    Study Guide 2 . The SN2 Mechanism: One Step, One Motion In an SN2 reaction, the alkyl halide has a polar carbon – halogen bond . The mechanism occurs in a single, concerted step . Figure 1 The S N 2 mechanism can also be illustrated as shown in Figure 2. Figure 2 How It Happens • The nucleophile attacks the carbon atom from the back side , opposite the leaving group • This attack occurs on the antibonding (back) lobe of the carbon orbital • A short - lived activated complex (transition state) forms, where the carbon is temporarily bonded to both the nucleophile and the leaving group • As the leaving group departs, the new bond forms simultaneously

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    Study Guide 3. Geometry and Inversion of Configuration • In the transition state, the carbon atom briefly adopts a trigonal bipyramidal shape • After the leaving group exits, the carbon returns to a tetrahedral shape • However, the spatial arrangement of groups around carbon is inverted This inversion is called the Walden inversion and is a defining feature of SN2 reactions. Figure 3 Why Backside Attack Is Required The nucleophile must attack from the side opposite the leaving group because: • The front side is blocked by the leaving group • The antibonding orbital is accessible only from the back As a result, front - side attack does not occur in SN2 reactions. 4. Steric Hindrance in SN2 Reactions SN2 reactions are very sensitive to steric hindrance . • If many or bulky groups surround the reacting carbon, the nucleophile has difficulty approaching • Increased crowding slows the reaction or stops it entirely

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    Study Guide Reactivity Trend • Methyl (CH ₃ – X): fastest • Primary alkyl halides: fast • Secondary alkyl halides: slow • Tertiary alkyl halides: no reaction Bulky groups create greater steric hindrance and reduce the reaction rate. 5. Solvent Effects on SN2 Reactions Protic Solvents • Can form hydrogen bonds • Strongly solvate nucleophiles • Lower nucleophile reactivity • Decrease SN2 reaction rate Polar 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 rate Lower activation energy leads to faster reactions. Figure 4 illustrates the effect of solvent polarity on the energy of activation and, thus, the rate of reaction. Figure 4 6. The SN1 Mechanism: Two Steps, One Intermediate The SN1 mechanism occurs in two separate steps . Step 1: Formation of the Carbocation (Slow Step) • The alkyl halide breaks apart • A carbocation and a leaving group anion are formed • This step determines the reaction rate Step 2: Nucleophilic Attack (Fast Step) • The nucleophile attacks the carbocation • A new bond forms to give the substitution product

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    Study Guide Because only the substrate is involved in the slow step, the reaction is unimolecular . 7. Carbocations and Optical Activity • Carbocations are sp² - hybridized and planar • The nucleophile can attack from either side of the plane As a result: • An optically active starting material produces a racemic mixture • Equal amounts of both enantiomers are formed This loss of stereochemical information is a hallmark of SN1 reactions. Figure 5 Key Takeaway • Nucleophilic substitution occurs by SN2 or SN1 mechanisms • SN2 reactions are:

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