In your own words, what is the major difference in the addition of a Grignard reagent to an oxidation state III carbonyl (ester/acid chloride) versus an oxidation state II carbonyl? (aldehyde/ketone)
Oxidation state III carbonyls (esters, acid chlorides) contain a built-in leaving group (such as -OR or -Cl) and so undergo nucleophilic acyl substitution reactions. The product is another carbonyl.
Oxidation state II carbonyls (aldehydes and ketones) do not contain a built-in leaving group and so undergo nucleophilc acyl addition reactions (instead of substitution). The product is an alcohol.
Because Grignards react with all carbonyls- esters and aldehydes/ketones- esters and acid chlorides will react twice with Grignards: once in a Nuc Acyl Sub mechanism to form a ketone, which will then react with another equivalent of Grignard in a Nuc Acyl Add mechanism to form an alcohol.
MendelSet practice problem # 669 submitted by Matt on July 18, 2011.
For the molecule shown below, indicate the hybridization (sp3, sp2, sp, etc.) of atoms A through H, and the bond angles of X, Y, and Z.
The trick to this problem is to look for implicit hydrogens- hydrogens (protons) that are not drawn in, but assumed to be there because there are no charges.
Also remember these rules:
If an atom has no multiple bonds, the total number of "things" (bonds and lone pairs) surrounding the atom is probably four, so it's sp3 hybridized (s1 + p3 = 4).
If an atom has one double bond, the total number of "things"surrounding the atom is probably three, so it's sp2 hybridized (s1 + p2 = 3).
If an atom has one triple bond or two double bonds, the total number of "things" surrounding the atom is probably two, so it's sp hybridized (s1 + p1 = 2).
Asp2 This oxygen has one double bond and two lone pairs. Three things total = sp2.
Bsp2 This carbon has one double bond and two single bonds, to a carbon and an oxygen (so two bonds). Three things total = sp2.
Csp3 This nitrogen has one long pair, two implicit hydrogens (two bonds), and one single bond to a carbo (another bond). Four things total = sp3.
Dsp3 This carbon has one implicit hydrogen and three single bonds. Four things total = sp3.
Esp3 This oxygen has two lone pairs and two bonding pairs. Four things total = sp3.
Fsp2 This carbon has one double bond (one bond), one implicit hydrogen (another bond), and is bonded to one other carbon (a third bond). So three bonds in total. Three things total = sp2.
Gsp This carbon has a triple bond (one bond) and a single bond (another bond). So two things in total = sp.
Hsp This nitrogen has a lone pair and a triple bond. Two things total = sp.
For angles, remember that electrons repel each other, and so will try to be as far apart from each other as possible:
Best way to separate two things is a straight line (180°).
Best way to separate three things is a "trifecta" (120°).
Best way to separate four things in three dimensional space is a tetrahedron (109.5°).
X 120° The carbon between the two oxygens is sp2 hybridized, so the shape is trigonal planar and the bond angle is 120°.
Y 180° The carbon bonded to the nitrogen is sp hybridized, so the shape is linear and the bond angle is 180°.
Z 109.5° The carbon is sp3 hybridized (the most common type of carbon). So the shape is tetrahedral and the bond angle is 109.5°.
MendelSet practice problem # 1286 submitted by Matt on October 2, 2011.
Allylic and benzylic halides tend to undergo both SN1 and SN2 substitution reactions at a faster rate than their alkyl counterparts.
For example, both allyl chloride and benzyl chloride undergo SN2 reaction at a faster rate than propyl chloride.
The same holds true for SN1 reactions: a 2° allyl or benzyl halide undergoes SN1 reaction faster than a 2° alkyl halide. Explain.
There are separate explanations for SN1 and SN2.
SN1: The SN1 mechanism involves a carbocation intermediate, and both allylic and benzylic carbocations have resonance, which increases the stability of their carbocations, and speeds up the rate of SN1 reaction.
SN2: This explanation is less obvious, and is probably only mentioned in passing in your orgo textbook, if at all.
The pi systems present in allylic and benzylic halides are able to overlap with the pi orbitals of the nucleophile and the leaving group in the 5-coordinate transition state of an SN2 reaction. This orbital overlap lowers the energy of the transition state (which stabilizes it), and so increases the rate of SN2 reaction.
There are other possible explanations as well (Which also vary by textbook). For one, an allylic halide is less sterically hindered than an alkyl halide (less hydrogens sticking out in 3D).
Also, the carbons on the double bond of allyl and benzyl compounds are sp2 hybridized, and so are more electronegative than sp3 carbons. So the sp2 carbons pull alway some electron density from the alpha carbon (the carbon attached to the leaving group), making it more electrophilic, and thus increasing SN2 reactivity.
MendelSet practice problem # 1281 submitted by Matt on October 1, 2011.
a) Draw the structures of all possible monochloro products resulting from the free-radical chlorination of 2-methylbutane.
b) Based on statistics alone, what do you expect the major product to be? Is this the same structure as the expected major product? Explain.
c) How would the relative yield of the products differ if bromine was used instead of chlorine?
a) There are five different carbons in 2-methylbutane, so hypothetically, you should get five different products, 20% A, 20% B, 20% C, 20% D, and 20% E.
But notice that A and B are the same product! So only four products are possible.
b) Based on statistics alone, we would expect product A (or B) to be the major product, encompassing 40% of all the products (20% A + 20% B).
But this is not what happens in real life because A is primary, and for free-radical halogenation reactions, more substituted products are favored. So the major product will be C, which is tertiary.
This happens because the intermediate is a radical, and radical stability goes 3° > 2° > 1°.
c) Chlorine reacts faster than bromine, and so is less selective than bromine.
For example, we know that the 3° halide will be the major product. But with chlorine, the proportion might turn out to be ~70% 3° , 20% 2°, and 10% 1°.
But since bromine reacts more slowly, you will get better selectivity. The proportion of products will might be something like ~95% 3° , 4% 2°, and 1% 1°.
While on this topic of reactivity, it's interesting to note that iodine reacts too slowly to be useful, and fluorine reacts so violently it's dangerous!
MendelSet practice problem # 1280 submitted by Matt on October 1, 2011.
Rank each set of compounds in order of decreasing boiling point (1 = highest boiling point):
a) ethane, n-octane, n-pentane
b) n-butane, 1-butanol, 1-chlorobutane.
c) n-octane, 2-methylheptane, 2,5-dimethylhexane
(Note that the n- prefix before an alkane just means that it's one chain, without any branching.)
Remember the rules of what gives compounds a higher boiling point (and melting point):
Van der Waals Forces
More electrons = more Van der Waals interactions = higher boiling point
So in general, the more "stuff" in a molecule (the higher the molecular weight), the higher its b.p. Example: C10H22 will have a higher b.p. than C5H12.
How to remember this trend? What do you grill with? propane gas. (C3H8)
What's in a lighter? butane liquid (C4H10)
Hydrogen Bonding
Compounds that can hydrogen bond have higher b.p./m.p/ than those that don't.
Compounds can hydrogen bond is they have a N, O, or F bonded directly to an H.
In organic chemistry, the best examples are alcohols (ROH) and amines (RNH2).
Branching
Branching makes it harder for molecules to pack together, which makes it harder to form Van der Waals interactions, and so tends to lower b.p./m.p.
a) (highest b.p.) n-octane > n-pentane > ethane (lowest b.p.)
Reason: octane has the most "stuff" (higher molecular weight).
b) 1-butanol > 1-chloroethane > n-butane
Reason: 1-butanol can hydrogen bond. 1-chloroethane has a higher molecular weight than n-butane.
c)n-octane > 2-methylheptane > 2,5-dimethylhexane
Reason: Branching. n-octane has no branching. 2,5-dimethylhexane has the most branching. Notice that each compound has the same molecular formua- C8H18. (and therefore the same weight).
MendelSet practice problem # 1287 submitted by Matt on October 2, 2011.
Propose a synthesis to accomplish each transformation. The only carbon sources allowed are alkenes and NaCN.
a) The starting alkene has 4 carbons, and the product has 5 carbons and a nitrogen. So you have to add 1C and 1N. This screams "add cyanide!"
Based on the location of the alcohol on the product, it looks like the way to go is to add NaCN to an epoxide. This puts the -CN and the -OH in the right positions.
b) The best way to prepare 2º and 3º amines is via reductive amination. Working backwards, the product can be prepared from a 4-carbon aldehyde (butyraldehyde) and a 3-carbon amine (propyl amine).
Performing hydroboration on the butene results in the anti-Markovnikov alcohol, which can be oxidized to the aldehyde using PCC.
To prepare propyl amine from an alkene, add HBr/peroxides to make the bromide (anti-Markovnikov), and then add NaN3 follow by H2/Pd.
MendelSet practice problem # 757 submitted by Matt on July 27, 2011.
For each amine below, show all Hofmann elmination products.
If more than one product is formed, predict which one will be the major product.
The Hoffman elimination first turns an amine into a better leaving group by methylating it with excess methly iodide, and then eliminates the methylated amine using base (such as -OH fomed from Ag2O in H2O). The products are alkenes.
This reaction is similar to an E2 elimination in that a beta proton is ripped off, and the alkene forms between alpha and beta. One notable difference from the E2 is that the less substituted product is favored, so Hoffman products are usually anti-Zaitsev.
a) There are three different types of beta protons, so three possible products can form. Product A is the least substituted alkene, so it's the major product.
b) There is only one possible product. The Hoffman elimination can't be done on aryl hydrogens (benzene rings).
MendelSet practice problem # 756 submitted by Matt on July 27, 2011.
The nitrosyl cation is shown below. Also shown are several proposed resonance arrows, only one of which is correct.
Draw the resonance forms that would follow from each set of arrows, and include formal charges. Which one is the correct resonance form? Explain your reasoning.
When evaluating possible Lewis structures the most important rule to follow is the octet rule: every atom must be surrounded by 8 and only 8 electrons (atoms in period 3 or below on the periodic table have d orbitals and so can contain more than 8 electrons, but that exception does not apply here).
Do any of the resonance forms drawn have atoms that break the octet rule? Yes- they all do! Except for resonance arrows in D. So D is the correct resonance form.
MendelSet practice problem # 755 submitted by Matt on July 27, 2011.
Show how each amine can be prepared from a carbonyl and an amine via reductive amination.
Reductive amination is a two step process. First, an imine or enamine is formed from a carbonyl and an imine.
Then, a the imine (or enamine) is reduced to an amine (using reducing agents such as H2, NaBH4, NaBH3(CN), NaBH(OAc)3, etc.). For practice on forming imine and enamines, see problem 712).
MendelSet practice problem # 754 submitted by Matt on July 27, 2011.
Rank the amines W through Z below in order of decreasing basicity (1 = most basic). Explain your reasoning.
Electron withdrawing groups (EWG) reduce electron density and reduce basicity. Resonance delocalizes electron density and so also decreases basicity (the electrons are less available to pick up a proton).
Electron donating groups (EDG) add electron density and increase basicity.
Alkyl groups are weakly EDG (induction), and hydrogen is neither EDG nor EWG. So cyclohexylamine (compound X) is a stronger base than ammonia (compound W).
Aniline (compound Y) has resonance, so it is less basic than compound X, which does not have resonance.
N-acyl aniline (compound Z) has more resonance forms than aniline, so it will be the least basic.
So the overall over is:
(most basic) X > W > Y > Z (least basic)
MendelSet practice problem # 753 submitted by Matt on July 27, 2011.
Rank the amines A through D below in order of decreasing basicity (1 = most basic). Explain your reasoning.
sp2 carbons are more electronegative (electron withdrawing) than sp3 carbons due to increased s-character. Electron withdrawing groups (EWG) make acids more acidic and bases less basic.
For this reason, piperidine (compound A) is the more basic than pyridine (compound B):
(more basic) A > B (less basic)
Nitro (-NO2) is an EWG, so 4-nitropyridine (compound C) will be less basic than compound A.
Dimethylamine is an electron donating group (EDG), so it adds electron density to pyridine and increases basicity. So dimethylamino pyridine (DMAP, compound D) will be more basic than compound A:
(more basic) D > B > C (less basic)
So how does compound A compare with compound D? There is no way of predicting this based on EWG/EDG rules alone. After all, what should be more basic: piperidine, which has sp3 carbons (increases base strength), or DMAP, which has sp2 carbons (decreases base strength) but also an EDG (increases base strength) ?
There's no way to tell. But it turns out that Piperidine is a little more basic than DMAP. So the overal order is:
(more basic) A > D > B > C (least basic)
MendelSet practice problem # 752 submitted by Matt on July 27, 2011.
Alpha bromination is usually carried out under acidic conditions via the enol intermediate.
Alpha bromination is uncontrollable under basic conditions, which goes through the enolate intermediate. Let's explore why.
a) Rank each carbonyl below in order of decreasing alpha-proton acidity (1= most acidic). Explain.
b) based on a), why does the reaction below lead to polyhalogenation?
a) Bromine is an electron withdrawing group (EWG), which makes nearby protons more acidic. So the carbonyl with two bromines is the most acidic.
b) To perform alpha bromination, the enolate (or enol) must first be formed, which then attacks Br2.
The problem with doing this under basic conditions is that each successive bromination leads to a carbonyl that is more acidic, and so forms an enolate even more easily. So once this reaction starts, it's difficult to control.
Enols are less nucleophilic than enolates, so this react can be controlled under acidic conditions. (Br2/H3O+ instead of Br2/NaOH).
MendelSet practice problem # 748 submitted by Matt on July 27, 2011.