CHEM 281 FINAL EXAM QUESTIONS WITH CORRECT ANSWERS LATEST UPDATE 2026

CHEM 281 FINAL EXAM QUESTIONS WITH CORRECT ANSWERS LATEST UPDATE 2026 Why does an alkene act as a nucleophile? - Answers It's pi bond is relatively unstable, and can donate electrons to an electrophile. What is the trend for alkene reactivity in addition reactions and why? - Answers More substituted alkenes react faster because of hyper conjugation, they donate electron density towards the pi bond making it more electron rich Also more submitted alkenes form more stable carbocations, which are involved in many addition reactions What is the order of reactivity for hydrogen halides to be added to a double bond? - Answers Reactivity = acidity HI HBr HCl HF What is the mechanism of addition of a hydrogen halide? Draw it. - Answers 1. Pi bond electrons attack the H, which separates from the X 2. H attaches to less substituted carbon, forming carbocation on other carbon 3. X- attacks other carbon 4. Alkyl halide formed What is the stereoselectivity and regioselectivity of hydrogen halide addition? - Answers Stereo: none, racemate formed because attack is on a carbocation Regio: Markovnikov Explain what the Markovnikov product is and why it is favoured in most addition reactions. - Answers When the first atom/group added is put onto the less substituted carbon, this occurs because of carbocation stability, it is better for the positive charge to be on the MORE substituted carbon Explain the mechanism of acid-catalyzed hydration. Draw it. - Answers 1. Acid forms H3O+ from water molecules 2. Pi bond of alkene attacks H of H3O+, adds to less substituted carbon 3. Carbocation formed - rearrangement possible here 4. Water attacks carbocation, forming alcohol What is the stereoselectivity and regioselectivity of acid-catalyzed hydration? - Answers Stereo: none, racemate formed because attack is on a carbocation Regio: Markovnikov How can alcohol be produced from a reaction with CONCENTRATED acid (not acid-catalyzed hydration)? - Answers 1. Pi bond attacks H on the acid, forming carbocation 2. A- attacks carbocation, forming a substitution product 3. Heat the mixture, add water 4. A- will depart, re-forming carbocation 5. H2O can attack to form alcohol What is the primary use of oxymercuration-demurcuration? - Answers Avoids rearranged products because no full carbocation is formed Explain the mechanism of alcohol production via oxymercuration-demercuration. Draw it. - Answers 1. Pi bond attacks Hg on Hg(OAc)2, causing departure of one OAc 2. Hg+ added to less substituted carbon, but forms a bridging structure between both carbons of the DB, so NO full carbocation 3. H2O attacks more substituted carbon, breaks apart bridging structure 4. NaBH4 added to cleave C-Hg bond and replace it with H What is the stereoselectivity and regioselectivity of oxymercuration-demercuration? - Answers Stereo: none Regio: Markovnikov How can the oxymercuration-demercuration process be modified to form an ether? - Answers Instead of using THF/H20, use THF/alcohol, then the product formed will be an ether What is the primary use of hydroboration-oxidation? - Answers Allows for production of anti-Markovnikov product Explain the mechanism of just hydroboration. Draw it. - Answers 1. At the same time, pi bond electrons attack the empty p orbital on BH3 and H on BH3 attacks the more substituted carbon 2. Tight, 4-membered ring intermediate formed with BH2 attaching to less substituted carbon 3. Process can repeat two more times 3. Trialkyl borane, where B is attached to 3 alkyl groups at the less substituted position, is formed Explain the mechanism of oxidation of a trialkyl borane. Draw it. - Answers 1. NaOH and H2O2 (peroxide) are added to the mixture 2. OH- abstracts a proton from H2O2, forming HOO-, a very reactive nucleophile 3. HOO- attacks B, forming a B- intermediate 4. The B-O bond is weak, and so an alkyl shift occurs where one alkyl group migrates onto the O 5. This repeats 3 times, forming a trialkyl borate ester 6. OH- attacks the B, causing departure of one of the alkyl ester groups, making an alkoxide anion 7. Alkoxide anion removes a proton from the OH, producing the alcohol Explain why the addition of BH3 occurs the way it does. - Answers H- from BH3 attacks the more substituted carbon because it has more positive charge built up. Also, it is easier for BH2 to bond with the less substituted carbon because of sterics. How does the alkyl shift that occurs in trialkyl borane oxidation change stereochemistry? - Answers It doesn't, it migrates with retention of stereochemistry What other process can occur with trialkyl boranes and what does this form? Draw the mechanism. - Answers Can protolyze them instead of oxidizing them, which is another way of essentially hydrogenating the original alkene 1. Add carboxylic acid 2. O in acid attacks B, forming a B- intermediate 3. Alkyl shift occurs onto the H, which breaks its bond to the O What is the stereoselectivity and regioselectivity of hydroboration-oxidation? - Answers Stereo: syn addition Regio: anti-Markovnikov What is the stereoselectivity and regioselectivity of hydroboration-protonolysis? - Answers Stereo: syn addition Regio: anti-Markovnikov, but not really obvious because both groups end up being H's anyways Why do molecular halogens react with alkenes? - Answers When they approach a double bond, the electrons in it can cause an induced dipole that makes one X a little bit positive and one X a little bit negative, making it somewhat electrophilic. Also, the X-X bond is weak to begin with. What unique structure is formed during molecular halogen addition? Who discovered it and how? - Answers Cylic halonium intermediate Was discovered by Queen's prof R.S Brown, by using a very large bi-cyclic ring molecule This ended up forming a stable intermediate that could not undergo nucleophilic attack because the groups were too large, so it was able to be isolated Explain the mechanism of molecular halogen addition in a non-nucleophilic solvent. Draw it. - Answers 1. Pi bond electrons attack the delta-positive X, which separates from the other X 2. Cyclic halonium intermediate formed 3. X- attacks more substituted carbon of the intermediate, it breaks open 4. Vicinal di-halide formed What is the stereoselectivity and regioselectivity of addition of molecular halogens? - Answers Stereo: anti addition Regio: more substituted carbon attacked What happens if molecular halogen is added in nucleophilic solvent like water? - Answers Halo alcohol or halohydrin is formed instead of vicinal dihalide Mechanism is essentially identical except instead of X- attacking, H2O does Why does the cyclic halonium intermediate form at all? - Answers X is large enough to bridge the gap and its large size makes it good at stabilizing positive charge What is an important use of halohydrins/haloalcohols? Explain the mechanism for it. - Answers Can be used to form epoxides via an internal SN2 reaction 1. Addition of one equivalency of base de-pronates OH 2. O- attacks the carbon attached to the halogen, forming a cyclic epoxide Which is favoured, an internal or external nucleophile, and why? - Answers Internal - higher effective concentration and entropy How do alkynes react with molecular halogens? - Answers Exact same way as alkenes, but can selectively use 1 equivalency to stop at the alkene If you do this, trans product will be formed due to anti addition How do alkynes react with HX? - Answers Exact same way as alkenes, but can selectively use 1 equivalency to stop at the alkene Why is the second step of an alkyne reaction with excess HX or molecular halogen slower? - Answers Once a halogen is added on, it draws electron density away from the pi bond making it less electron rich and less nucleophilic Explain the mechanism of OsO4 addition to an alkene. Draw it. - Answers 1. At same time, pi bond electrons attack Os and O attacks carbon of alkene (concerted cycloaddition) 2. Ring intermediate formed 3. H2O hydrolyzes, producing a diol What is the stereoselectivity and regioselectivity of addition of OsO4? - Answers Stereo: syn addition (cis and trans will give different isomers!) Regio: none, both C's are getting OH's Why does OsO4 react in the first place? - Answers It contains osmium 8, which is in a high oxidation state and wants to receive electrons to reduce down to osmium 6 What is NMO? - Answers Co-oxidant that is often added to the OsO4 reaction because OsO4 is expensive and it allows for recycling of it How can epoxides be formed from alkenes? - Answers Addition of organic peroxide/peracid, such as mCBPA What is the stereoselectivity and regioselectivity of epoxide formation from alkene/peracid? - Answers Stereo: syn addition (cis & trans will give different isomers!) Regio: none, both C's are ultimately getting attached to O How can a trans diol be produced from an alkene? - Answers Form an epoxide with mCPBA or another peracid, then do an acid-catalyzed reaction with water What occurs in the reaction of alkenes with KMnO4? What products are formed? - Answers Aggressive oxidation - double bonds are cleaved Products depend on degree of substitution around the double bond 1. fully substituted = ketone 2. one hydrogen = acid 3. two hydrogens = diacid which will immediately turn into CO2 and H2O What occurs in the reaction of alkenes with O3? What products are formed? - Answers Mild oxidation - double bonds are cleaved Products depend on the degree of substitution around the double bond 1. fully substituted = ketone 2. one hydrogen = aldehyde 3. two hydrogens = formaldehyde What occurs in the reaction of alkynes with KMnO4? - Answers Carboxylic acid formed What occurs in the reaction of alkynes with O3? - Answers Carboxylic acid formed What is the Bayer's test for unsaturation? - Answers Oxidation of an alkene with KMnO4 produces brown precipitate MnO2 as a byproduct, so if this colouring is detected, you know an alkene was present List all the methods we have for synthesizing alcohols from alkenes (6 ways) - Answers 1. Acid-catalyzed hydration 2. Oxymercuration-demercuration 3. Hydroboration-oxidation 4. OsO4 oxidation (cis diol) 5. Epoxide formation with peracid, then acid-catalyzed reaction with water (trans diol) 6. Molecular halogen in water (halo alcohol) What are the two overarching methods for converting alcohols to good leaving groups? - Answers Converting to a sulfonate ester or converting to a halide Name but do not explain the methods of converting an alcohol to an alkyl halide. - Answers 1. HX directly 2. Another acid, and a salt of X- 3. PBr3 4. SOCl2 Explain the mechanism of converting an alcohol to an alkyl halide using HX directly, mentioning any stereochemical/regioselective requirements. Draw it. - Answers 1. O of OH attacks H of HX, becoming protonated 2. Now, mechanism depends on substitution - if alcohol is primary, X- will attack in SN2 (stereospecific), if alcohol is secondary or tertiary, X- will attack in SN1 (racemate) What is the reactivity trend for conversion of an alcohol to an alkyl halide using HX directly? - Answers More substituted alcohols are more reactive, and more acidic HX's (HIHBrHCl, HF is unreactive) Explain the mechanism of converting an alcohol to an alkyl halide using another acid and a salt of X-, mentioning any stereochemical/regioselective requirements. Draw it. - Answers 1. O of OH attacks H or other Lewis acid, becoming protonated or positive 2. Now, mechanism depends on substitution - if alcohol is primary, X- will attack in SN2 (stereospecific), if alcohol is secondary or tertiary, X- will attack in SN1 (racemate) Explain the mechanism of converting an alcohol to an alkyl halide using PBr3, mentioning any stereochemical/regioselective requirements. Draw it. - Answers 1. O of OH attacks P of PBr3, causing departure of one Br 2. Br- attacks C attached to O in an SN2 fashion (stereochemical inversion) 3. HOPBr2 departs and can be used 2 more times, ultimately forming 3 alkyl halide equivalencies and P(OH)3 Explain the mechanism of converting an alcohol to an alkyl halide using SOCl2, mentioning any stereochemical/regioselective requirements. Draw it. - Answers 1. O of OH attacks S of SOCl2, causing departure of one Cl 2. Weak base (pyridine) removes H from O