ACS Organic Chemistry Exam

  What is the general formula for a carbohydrate? - Correct Answers: Cx(H2O)y How do you determine the number of stereoisomers for a given carbohydrate? - Correct Answers: 2^n where n= number of stereocenters Distinguish between the D and L designations for carbohydrates. - Correct Answers: In the L form the hydroxy group is on the bottom left closest to the CH2OH group in the frame of a Fischer projection In the D form the OH is on the bottom right What are anomers? - Correct Answers: Diastereomers formed by the cyclization of carbohydrates (*note they have different configurations at the new stereocenter-this is what makes them diastereomers) Distinguish between alpha and beta anomers. - Correct Answers: beta anomers have the OH belonging to the hemiacetal carbon equatorial alpha anomers have the OH belonging to the hemiacetal carbon axial What is the definition of mutarotation? - Correct Answers: the process for which alpha and beta anomers come to equilibrium (aqueous) in a specific ratio. It is the same ratio is observed when you start with the alpha anomer in water versus the beta anomer in water What agent is most often used for complete oxidation of a carbohydrate? - Correct Answers: HNO3 What kind of agents are used for selective or partial oxidation of carbohydrates? - Correct Answers: most commonly Br or some other MILD oxidative agent What are reducing sugars? - Correct Answers: Carbohydrates that yield positive results of the oxidative tests, because in being oxidized they reduce the metal ion (or other reducing agent) Carbohydrates undergo reactions similar to which functional group? - Correct Answers: OH (hemiacetals and acetals) How does the Tollen's test work? - Correct Answers: Carbohydrate is oxidized by Ag+ and positive result shows a silvery mirror as Ag+ is reduced to metallic (elemental) silver How do the Benedict's and Fehling's tests work? - Correct Answers: Both employ a solution of aqueous Cu2+ in base. When reduced it forms Cu2O which is a red ppt. What is a glycoside? - Correct Answers: a sugar bonded to another oxygen containing functional group to make the carbohydrate a full acetal rather than a hemiacetal What does a glycosidic bond consist of? - Correct Answers: A glycosidic bond is between C4 (the acetal carbon) and another oxygen atom What is the difference between an alpha and a beta glycosidic bond? - Correct Answers: the alpha bond is axial the equatorial bond is equatorial What is a nucleophile? - Correct Answers: An electron rich species than seeks an electron poor site What is an electrophile? - Correct Answers: An electron poor species which seeks an electron rich species A nucleophile is, by definition what other classification of species? - Correct Answers: Lewis base What is the main difference, in terms of how the reaction occurs, between Sn1 and Sn2 reactions? - Correct Answers: Sn1- the leaving group leaves then the nucleophile comes in Sn2- the leaving group leaves as the nucleophile is coming in (simultaneously) What stereochemistry do Sn2 reactions follow? - Correct Answers: inversion of configuration (so from R to S or S to R) *Note inversion is the mirror image What are the main differences between Sn1 and Sn2 reactions? - Correct Answers: Sn2- more substituents hinder, will not happen for tertiary carbons, polar, protic solvents, inversion of configuration Sn1- more substituents help, will not happen for primary carbons, forms carbocation, aprotic solvents, racemization (or else slightly more inversion of configuration than retention) Do leaving groups affect the rates of Sn1 or Sn2 reactions? - Correct Answers: Yes. Both. What are general trends to determine the strength of a leaving group? - Correct Answers: Going down a group on the periodic table increases the strength of the leaving group, if the leaving group is stable on its own, it is more likely to leave; strong bases make poor leaving groups What are some of the trends for determining the strength of a nucleophile? - Correct Answers: increasing base strength is proportional to the strength of the nucleophile, as you go down a column on the periodic table the nucleophile gets weaker, steric bulk decreases ability of nucleophile Does the identity of the nucleophile have an effect on Sn1 or Sn2 reactions? - Correct Answers: The identity of the nucleophile has no effect on the rate of Sn1 reactions because the nucleophile adds after the rate determining step. What are the 3 most common aprotic solvents? - Correct Answers: dimethylsulfoxide (DMSO), dimethylformamide (DMF), acetone What are the 5 most common protic solvents? - Correct Answers: water, ethanol, acetic acid, methanol, trifluoroacetic acid When are Sn2 reactions favored? - Correct Answers: aprotic solvents, strong nucleophile, primary substrates When are Sn1 reactions favored? - Correct Answers: stable carbocation (tertiary substates), poor nucleophiles, good leaving group, polar solvent What is an elimination reaction? - Correct Answers: When the leaving group and a proton on an adjacent carbon both leave to form a double bond What is the term for the most basic type of eliminations? - Correct Answers: 1,2-eliminations (because proton and leaving group are on adjacent atoms); this can also be called beta-elimination What, physically, happens in an E2 elimination? - Correct Answers: Involves breaking and making several bonds all at once; the nucleophile will start to bond (taking the proton), the leaving group leaves and the double bond starts to form (remember all at once) What do E2 eliminations typically compete with? - Correct Answers: Sn2 What is Zaitzev's Rule and in which kinds of reactions does it apply? - Correct Answers: the product will be the more substituted compound; applies to elimination reactions-note, of course there are exceptions What is the one "major" exception to Zaitzev's principle? - Correct Answers: Hofmann eliminations What does oxidation signify in organic chemistry? - Correct Answers: results in an overall increase in the number of oxygens or a decrease in hyrdogen content What does reduction signify in organic chemistry? - Correct Answers: results in a decrease in oxygen content or an increase in hydrogen content What happens when you oxidize a primary alcohol? - Correct Answers: goes to an aldehyde and then straight to a carboxylic acid unless it is stopped What happens when you oxidize a secondary alcohol? - Correct Answers: ketone What happens when you oxidize a tertiary alcohol? - Correct Answers: nothing- it is inert Distinguish between regiospecific and regioselective. - Correct Answers: A regiospecific reaction is one wherein the reaction produces predominantly one possible orientation; regioselective means it forms some of both What is Markovnikov's rule? - Correct Answers: In reactions of HX to alkenes the H bonds to the carbon with more hydrogens (fewer substituents) and the X bonds to the carbon with more substituents (fewer hydrogens). This is for additions to alkenes/alkynes What is the mechanism rule? - Correct Answers: in additions to alkenes/alkynes the electrophile adds to form the most stable carbocation (more widely applicable-note this also accounts for rearrangement) What is a halohydrin? - Correct Answers: some species with both an OH group and a halogen bonded to 2 adjacent carbons What is a carbene? - Correct Answers: A carbon with only 2 bonds and an unshared pair- the simplest is CH2 What is Huckles rule? - Correct Answers: 4n+2 this tells us the number of electrons required for aromaticity What are the requirements for aromaticity? - Correct Answers: Is it flat? Cyclic? Conjugated? Huckle's number of electrons What is the difference between anti-aromatic and non-aromatic (which is the same as neither aromatic nor antiaromatic)? - Correct Answers: Anti-aromatic compounds do not have the correct number of electrons, but they do meet all of the other criteria How do we deal with peripheral aromaticity? - Correct Answers: rearrange the bonds such that they are all on the outside and there are no internal double bonds (on shared sides of a multi-ring system) What is an arenium ion? - Correct Answers: a benzene ring missing a hydrogen, giving one of the carbons a negative charge What is an arene? - Correct Answers: a ring structure with conjugated double bonds What is a phenol? - Correct Answers: an alcohol bonded directly to an aromatic ring What are the numerical designations for meta, ortho and para? - Correct Answers: meta- 1,3 ortho- 1, 2 para- 1,4 What does the ending "-one" represent? - Correct Answers: ketone What is the ending (naming) for aldehydes? - Correct Answers: -al What is an acyl chloride? - Correct Answers: cl-c=o Distinguish between the following groups: amide, nitro and nitrile - Correct Answers: amide- C=O-NH2 nitro- NO2 nitrile- C triple bond N What are the 3 main disadvantages of Friedel-Crafts alkylations? - Correct Answers: 1) the ending material is activated, meaning it is more reactive than the starting material (one of the few which adds activating group) and multiple substitution can be a problem 2) rings with moderate-strong deactivating groups cannot be alkylated 3) rearrangements What is a diazo group? - Correct Answers: N=N What is an arylamine? - Correct Answers: an aromatic ring (in our case nearly always benzene) with an NH2 group What is a hydrate? - Correct Answers: A carbon with 2 OH groups What is a ylide? - Correct Answers: CH2=PPh3 What are imines? - Correct Answers: really just aldehyde/ketone but instead of the O it is an N What are imines reduced to? - Correct Answers: NH2 What compound reduces imines? - Correct Answers: NaBH3CN When is an enamine formed? - Correct Answers: reaction of a secondary amine with a ketone/aldehyde How are imines formed? - Correct Answers: reaction of a primary amine with an aldehyde/ketone What is an enamine? - Correct Answers: amino group bonded to one of the carbons of a C=C What is an acetal? - Correct Answers: a carbon which is bonded to 2 oxygens which are not OH What is a hemiacetal? - Correct Answers: a carbon bonded to 2 oxygens, one of which is an OH, the other is not What does an amide hydrolize to? - Correct Answers: carboxylic acid