CHEM 281 MIDTERM #2 EXAM QUESTIONS WITH CORRECT ANSWERS LATEST UPDATE 2026

CHEM 281 MIDTERM #2 EXAM QUESTIONS WITH CORRECT ANSWERS LATEST UPDATE 2026 What are the formulas for an alkane and cycloalkane? - Answers Alkane: CnH2n+2 Cycloalkane: Cn2n What is the primary source of alkanes? - Answers Petroleum What is catalytic cracking? - Answers A process of "chopping up" larger hydrocarbons into gasoline size (octanes) What is the trend for number of carbons and boiling point? - Answers More C's = higher BP All C atoms in alkanes are ____ hybridized. - Answers sp3 Distinguish between a butyl, sec-butyl, tert-butyl, and isobutyl group. - Answers Butyl - just attached on the end Sec-butyl - straight chain, attachment at 2nd C Tert-butyl - three branch structure Isobutyl - 2 C's in line, and 2 attached at end What does a neopentyl look like? - Answers 2 carbons in line and three branched at the end More globular alkanes have a ______ boiling point than straight chain. - Answers Lower What is the most stable conformation of a Newman projection? - Answers Largest groups farthest apart EXCEPTION: If there is an opportunity for H-bonding, such as the presence of two OH groups, it is more stable for them to be side by side (gauche position) Distinguish between anti, gauche, and eclipsed Newman conformers. - Answers Anti - groups directly opposite (180° separation) Gauche - groups side by side (60° separation) Eclipsed - groups right overtop (0° separation) What can be deduced about a molecule's complexity from its conformer free energy diagram? - Answers The more different-height peaks the diagram has, the more complex it must be because of different groups attached What are the two components of ring strain? - Answers Angle strain - deviation from ideal bond angles Torsional strain - dispersion forces of all ring systems What is the trend for ring stability in terms of # of carbon atoms? - Answers More C's = more stable, with cyclohexane being exceptionally stable because it is the first ring structure in which all bonds can be perfectly staggered What are the two conformations of cyclohexane and which is more stable and why? - Answers Boat - two sides folded up Chair - one side peaked up, one side pushed down Chair is much more stable because of the staggered groups; in boat, they are all eclipsed AND there are 1,4 flagpole interactions Distinguish between axial/equatorial positions on cyclohexane and which are preferred. - Answers Axial - up/down straight Equatorial - out sideways from plane of ring Large groups prefer to be equatorial. What is a ring flip? - Answers Switch from one chair conformation of cyclohexane to the next - no bonds broken, all axial groups go equatorial and all equatorial groups go axial How is cis/trans designated on cyclohexane? - Answers With no regard for axial or equatorial, just considering whether groups are angled up or slightly up, or down or slightly down How do you name a bicyclic alkane? - Answers Start counting at the bridge in the direction of the longer side, and count the bridge last State name as bicyclo[long e]___ane What is hydrogenation and how is it used? - Answers Addition of H2 with heat/pressure and a metal like Pt, Ni, Pd catalyst Can be used to get alkanes from alkenes and alkynes (alkynes = 2 molar equivalents needed) What is index of hydrogen deficiency and how is it calculated? Include presence of other atoms. - Answers Difference in # of H2 a molecule is missing from its "parent" alkane Calculated by taking molecular formula and dividing difference in H's by 2 If... - oxygen is present, ignore it - halogen is present, treat it like an H - nitrogen is present, subtract one H and ignore it What does having a certain index of hydrogen deficiency tell us? - Answers IHD's come from 2 places - rings, and double/triple bonds. Therefore, having one tells us that we deviate from the parent alkane by one of these things Distinguish between the terms chiral and achiral. - Answers Chiral = can not be superimposed on its mirror image, does not have a plane of symmetry within it Achiral = can be superimposed on its mirror image, does have a plane of symmetry within it Distinguish between constitutional and stereoisomers, and any sub-classes. - Answers Constitutional - different connectivity between atoms Stereoisomers - same connectivity, just different spatial relationships Sub-classes of stereoisomers: enantiomers (mirror images) and diastereomers (not mirror images) Explain the distinction between a diastereomer and a chiral molecule. - Answers A diastereomer MAY be chiral, but doesn't have to be. For example, cis & trans isomers of an alkene are diastereomers, but neither is chiral. Likewise, meso compounds are diastereomers of a molecule, but are not chiral. Explain the distinction between an enantiomer and a chiral molecule. - Answers An enantiomer HAS to be chiral by definition, because if it has a mirror image that is not super imposable, it must not be symmetrical and therefore will have optical activity. Distinguish between the properties of enantiomers vs diastereomers. - Answers Enantiomers have the same properties like BP, MP, etc Diastereomers have very different properties Distinguish between tetrahedral and trigonal stereogenic centres. - Answers Tetrahedral - at an sp3 carbon attached to 4 different groups, these are chiral Trigonal - at an sp2 carbon, not chiral What is the ultimate test of chirality? - Answers Plane of symmetry If you can draw a plane of symmetry within the molecule, it is NOT chiral; if you cannot, it is Briefly describe naming with the R-S system. - Answers 1. Order atoms by priority - based on atomic number of ATTACHED atom, then continue to point of difference 2. Rotate so that lowest priority is in back 3. Clockwise = R, counterclockwise = S How does one assign priority to a group that contains a multiple bond? - Answers Treat it as if each atom is duplicated or triplicated with single bonds What is the only property enantiomers differ in and how is it measured? - Answers Their ability to rotate plane polarized light Light is passed through a polarimeter through a pure sample of one enantiomer and a value called specific rotation (alpha) is calculated - enantiomers will have equal and opposite specific rotations What is a racemate? - Answers 50/50 mixture of the two enantiomers such that the mixture has no optical activity overall What is enantiomeric excess and how is it calculated? - Answers The proportion of a mixture that is pure one enantiomer 1. ee = (moles one E - moles other E)/total moles 2. ee = observed specific rotation/specific rotation of pure enantiomer What type of mixture has enantiomeric excess of 100%? - Answers Pure enantiomer How can we determine the (+) or (-) designation of an enantiomer for naming? - Answers You can't - only via experiments To form a pure enantiomer, one needs _____ starting materials. - Answers Chiral From achiral materials, you'll get a racemate How many possible stereoisomers exist for a given compounds? - Answers 2^n, where n is the number of chiral centres How can we distinguish whether molecules are diastereomers, enantiomers, or the same molecule in regular 3D structure drawings? - Answers Assign R/S configuration at each centre If it has changed in every centre, it is an enantiomer If it has changed at some centres, it is a diastereomer If it has not changed, it is the same molecule What is a meso compound? - Answers A compound with chiral centres that is not chiral overall because it has a plane of symmetry What are the rules for working with Fischer projections? - Answers Horizontal lines come out of the page Vertical lines go back into the page CANNOT pick a Fischer projection up off the page and rotate it, imagine it "glued down" How can we distinguish whether molecules are diastereomers, enantiomers, or the same molecule in Fischer projections? - Answers Easily - no R/S priorities needed If they look like mirror images, they're enantiomers If they look the same, they're the same If they look different but not mirror images, they're diastereomers What general protocol can be used to try to "get" all the stereoisomers? - Answers Draw one molecule, consider its mirror image Swap groups around one chiral centre, consider its mirror image How does stereoisomerism exist in cyclic compounds? - Answers Cis/trans are always stereoisomers of each other, so every cyclic molecule will have at least two stereoisomers Then, cis/trans may or may not have enantiomers themselves State the stereoisomers of 1,4, 1,3, and 1,2 disubstituted cyclohexanes (where substituents are the same). - Answers 1,4: cis & trans, but no enantiomers for either because both have a POS (∴ 2 stereoisomers) 1,3: cis & trans, trans has an enantiomer but cis has a POS (∴ 3 stereoisomers) 1,2: cis & trans, both have enantiomers but cis' rapidly ring flip back to their enantiomer because they are equal in energy, and therefore cannot be isolated as separate (∴ 3 stereoisomers) How do R/S configurations around carbons change when the cyclohexane ring flips? - Answers They don't - all stay the same, no bonds were broken What does it mean for 2 molecules to have the same relative configuration? - Answers 3/4 of their groups line up What is resolution and what are 2 methods of doing it? - Answers Separating two enantiomers in a mixture 1. Kinetic resolution - carry out a reaction in which one enantiomer reacts fast but the other reacts slow 2. Reacting with another chiral molecule, which will make diastereomers, which have different properties and can be separated Explain the chirality "exception" of two double-bonded carbons. - Answers If you have 2 double-bonded C's next to each other and each is attached to two different groups at the end (doesn't matter if groups are same on each carbon), then you will have a chiral molecule because the ends lie in different planes and it will NOT be superimposable on its mirror image. Distinguish between the electrophile, nucleophile, and leaving group. - Answers Electrophile - has an electron deficient carbon Nucleophile - has a lone pair (may be neutral or negative) Leaving group - group that is removed from the electrophilic carbon during substitution Name the 2 key features that differ between SN1 and SN2. - Answers SN1 - unimolecular rate law (only electrophile matters) and produces racemate (not stereospecific) SN2 - bimolecular rate law (electrophile and nucleophile matter) and produces one enantiomer (inversion of stereochemistry) Explain why the SN2 reaction has a bimolecular rate law. - Answers The reaction occurs in one step, so both the nucleophile and electrophile are involved in the transition state, which is what defines the rate law. Explain why the SN1 reaction has a unimolecular rate law. - Answers The reaction occurs in two steps, the first of which is slow & rate-determining, and this step involves only the electrophile forming a carbocation. The transition state therefore only has the electrophile, which is what defines the rate law. Explain why the SN2 reaction shows inversion of stereochemistry. - Answers Since the leaving group has not yet departed, all bonding orbitals on carbon are taken up with electrons, and the empty one that the nucleophile is able to donate to is the anti-bonding orbital which has most of its orbital density BEHIND the carbon. This leads to backside attack - the nucleophile must approach and form its bond there, and repulsion from this approach "sweeps" the other groups forward, leading to stereochemical inversion. Explain why the SN1 reaction produces both enantiomers of a given compound. - Answers The leaving group departs before the nucleophile attacks, which means the carbocation has a planar structure with a symmetrical p-orbital. The nucleophile can attack and donate to either side of this p-orbital with equal likelihood and therefore a racemate is obtained. How does the structure of the electrophile dictate SN1 vs SN2? - Answers Methyl, 1°, and 2° halides react by SN2 because they do NOT form a stable carbocation and do not have that much steric hindrance for direct attack. 3° halides react by SN1 because they DO form a stable carbocation and have a lot of steric hindrance to direct attack. Explain the phenomenon that causes increased substitution on a carbocation to stabilize it. - Answers Hyperconjugation Nearby carbon groups can donate electron density to the empty p-orbital on the carbon which helps to delocalize the charge How does bond strength relate to leaving group ability? - Answers Weaker bond = better leaving group Larger atoms have poorer orbital overlap and weaker bonds What is an absolute way of determining leaving group ability? - Answers Basicity The best leaving groups are the weakest bases, so look at pKa of conjugate acid - a lower pKa means a better leaving group What are the 2 ways we can convert an alcohol to a better leaving group? Draw both the mechanisms. - Answers 1. Protonation with an acid 2. Turn it into a sulfonate ester using a sulfonate chloride and pyridine. (see notes for mechanisms) What is an alkoxonium ion? - Answers Molecule that has had an OH group protonated so that it is O+ What makes sulfonate esters good leaving groups? Why triflate in particular? - Answers They have resonance stabilization after departing so the reaction is driven forward Triflate is particularly stable because the flourine atoms pull negative charge towards them, helping to disperse it even more What stereochemical change does conversion of an alcohol to a sulfonate ester involve? - Answers None! What is an epoxide and what is special about them? - Answers Cyclic ether They are special because normally oxygen is a very bad leaving group, but with so much ring strain in the epoxide, it is possible - starts off at high energy state so barrier is not as large Describe the 3 different conditions epoxides can react under. Draw the 3 mechanisms. - Answers 1. Good nucleophile, no acid - least substituted carbon will be attacked, forming only one product 2. Good nucleophile, acid - O will get protonated, then least substituted carbon will preferentially be attacked (major/minor product) 3. Weak nucleophile, acid - O will get protonated, then more substituted carbon will preferentially be attacked (major/minor product) (see notes for mechanism) Describe the stereochemistry of epoxide reactions. - Answers In ALL epoxide reactions, the attacked carbon will undergo inversion of stereochemistry, the other will retain its. How do epoxide reactions occur if both carbons are equally substituted? - Answers Racemate produced because both attacked equally