ACS Organic Chemistry Exam
Covering the full series of Orgo I and II plus lab for the ACS standard exam
Terms in this set (35)
What is the general formula for a carbohydrate?
How do you determine the number of stereoisomers for a given carbohydrate?
2^n where n= number of stereocenters
Distinguish between the D and L designations for carbohydrates.
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?
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.
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?
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?
What kind of agents are used for selective or partial oxidation of carbohydrates?
most commonly Br or some other MILD oxidative agent
What are reducing sugars?
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?
OH (hemiacetals and acetals)
How does the Tollen's test work?
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?
Both employ a solution of aqueous Cu2+ in base. When reduced it forms Cu2O which is a red ppt.
What is a glycoside?
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?
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?
the alpha bond is axial
the equatorial bond is equatorial
What is a nucleophile?
An electron rich species than seeks an electron poor site
What is an electrophile?
An electron poor species which seeks an electron rich species
A nucleophile is, by definition what other classification of species?
What is the main difference, in terms of how the reaction occurs, between Sn1 and Sn2 reactions?
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?
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?
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?
What are general trends to determine the strength of a leaving group?
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?
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?
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?
dimethylsulfoxide (DMSO), dimethylformamide (DMF), acetone
What are the 5 most common protic solvents?
water, ethanol, acetic acid, methanol, trifluoroacetic acid
When are Sn2 reactions favored?
aprotic solvents, strong nucleophile, primary substrates
When are Sn1 reactions favored?
stable carbocation (tertiary substates), poor nucleophiles, good leaving group, polar solvent
What is an elimination reaction?
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?
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?
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?
What is Zaitzev's Rule and in which kinds of reactions does it apply?
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?