Difficulty: Intermediate to Advanced | Prerequisites: Carbonyl chemistry fundamentals, acid/base equilibria, understanding of pKa, familiarity with Newman projections and stereochemistry.
Alpha carbon chemistry introduces the idea that hydrogens next to a carbonyl are acidic, which opens the door to enolate chemistry: a huge class of C-C bond-forming reactions. Meanwhile, carbohydrate chemistry applies everything you have learned about hemiacetals, stereochemistry, and conformational analysis to biological molecules. These two topics round out Exam 3 and bridge the gap between pure organic reactions and biochemistry.
Hydrogens on the carbon next to a carbonyl (alpha-H) are acidic because removing one gives a resonance-stabilised enolate. LDA gives the kinetic enolate (less substituted side); NaOEt/HOEt gives the thermodynamic enolate (more substituted side). For carbohydrates, learn the mechanical rules for converting a Fischer projection to a Haworth projection: right side goes down, left side goes up, and the anomeric OH position tells you alpha vs beta.
Alpha carbon (Cα)
The carbon atom directly adjacent to a carbonyl group. Its hydrogens are unusually acidic (pKa ~20 for ketones, ~25 for esters) because the resulting anion is stabilised by resonance with the carbonyl.
Alpha hydrogen (α-H)
A hydrogen bonded to the alpha carbon. Removal of this hydrogen by a base produces an enolate.
Enolate
The conjugate base formed when an alpha hydrogen is removed. It is a resonance-stabilised anion with negative charge shared between carbon and oxygen. The carbon end is nucleophilic and can form new C-C bonds.
LDA (lithium diisopropylamide)
A strong, sterically hindered, non-nucleophilic base. Used at low temperature (−78 °C in THF) to irreversibly deprotonate the less hindered alpha position, giving the kinetic enolate. Think of it as: the base that grabs the easiest-to-reach hydrogen and commits.
NaOEt/HOEt (sodium ethoxide in ethanol)
A moderate base that establishes an equilibrium between the carbonyl compound and its enolate. Under equilibrium (thermodynamic) conditions, the more substituted, more stable enolate predominates.
Kinetic enolate
The enolate formed at the less substituted alpha carbon. Favoured by LDA at −78 °C because the less hindered hydrogen is removed faster.
Thermodynamic enolate
The more substituted enolate, which is more stable (more substituted double bond). Favoured under equilibrating conditions (NaOEt/HOEt, room temperature).
Fischer projection
A 2D representation of a sugar's open-chain form. Horizontal bonds point toward the viewer; vertical bonds point away. The carbon chain runs top to bottom with C1 (the most oxidised carbon) at the top.
Haworth projection
A representation of the cyclic (ring) form of a sugar. The ring is drawn as a flat polygon (usually a six-membered pyranose or five-membered furanose) viewed from a slight angle, with substituents pointing up or down.
Anomer
The two possible stereoisomers at C1 (the anomeric carbon) when a sugar cyclises. In D-sugars: alpha has the anomeric OH pointing down in the Haworth projection; beta has it pointing up.
D-sugar
A sugar in which the OH on the highest-numbered chiral centre (C5 in a hexose) is on the right in the Fischer projection. The CH₂OH group points up in the Haworth projection.
Pyranose
A six-membered ring form of a sugar (five carbons and one oxygen in the ring), named after pyran.
The acidity of an alpha-H depends on how well the resulting enolate is stabilised.
More electron-withdrawing groups flanking the alpha carbon make the alpha-H more acidic.
Exam Q16 ranking: for a malonate diester (CH₃OOC-CH₂-COOCH₃), a beta-ketoester (RCOCH₂COOR'), and a simple carboxylic acid (RCOOH):
The beta-ketoester alpha-H (flanked by both a ketone and an ester) is the most acidic
The malonate diester alpha-H (flanked by two esters) comes next
The carboxylic acid OH is the least acidic in this particular ranking
The exam answer was: 3 > 1 > 2 (where 3 = the compound with the most electron-withdrawing stabilisation of the conjugate base)
LDA, THF, −78 °C → kinetic enolate:
LDA is bulky and strong; it irreversibly removes the most accessible (least hindered) alpha-H
The reaction is under kinetic control because at −78 °C there is not enough energy for the enolate to equilibrate to the more stable form
The product is the less substituted enolate
NaOEt, HOEt (room temperature) → thermodynamic enolate:
NaOEt is a weaker, less hindered base; the deprotonation is reversible
The system reaches equilibrium, and the more substituted enolate (more stable double bond, by the same logic as Zaitsev elimination) predominates
The product is the more substituted enolate
Exam Q17 showed a ketone treated first with LDA/THF/−78 °C (giving the kinetic enolate on the less substituted side) and then the same ketone treated with NaOEt/HOEt (giving the thermodynamic enolate on the more substituted side)
This is a mechanical, rule-based conversion. Learn the rules and apply them systematically.
Step 1: Number the carbons in the Fischer projection (C1 at the top for aldoses).
Step 2: Draw the Haworth ring. For a pyranose (six-membered ring), the ring oxygen sits at the back right, with C1 on the right and C5 on the left.
Step 3: Assign up/down positions for substituents at C2, C3, and C4:
Groups on the right side of the Fischer projection go down in the Haworth
Groups on the left side of the Fischer projection go up in the Haworth
Step 4: Determine the C5 substituent direction:
If the OH on C5 is on the right in the Fischer (D-sugar), the CH₂OH group at C6 points up in the Haworth
Step 5: Determine the anomeric position (C1):
Alpha anomer (for D-sugars): the OH at C1 points down (same side as the ring oxygen in Haworth)
Beta anomer (for D-sugars): the OH at C1 points up (opposite side from the ring oxygen)
The exam gave D-glucose in its Fischer projection and asked for the alpha-D-glucopyranose Haworth projection. Applying the rules: C2-OH down, C3-OH up (HO was on the left), C4-OH down, CH₂OH up, anomeric OH down (alpha).
Enolate chemistry is the basis for how living organisms build complex carbon skeletons. The Claisen condensation (enolate of an ester attacking another ester) is the biochemical logic behind fatty acid synthesis. Carbohydrate ring forms are how every sugar exists in solution and in biological macromolecules such as starch, cellulose, and DNA's sugar backbone.
Students often confuse kinetic and thermodynamic enolates. Remember: LDA at −78 °C gives the kinetic (less substituted) product. NaOEt at room temperature gives the thermodynamic (more substituted) product. If you mix these up, every enolate question goes wrong.
A frequent mistake in acidity rankings is assuming that the compound with the lowest pKa overall is the most acidic alpha-H. The question is specifically about the alpha hydrogens, not any OH groups on the molecule.
When converting Fischer to Haworth, students often put C3 substituents on the wrong side. If HO is on the left in the Fischer, it goes up in the Haworth. The mnemonic "right goes down" applies to C2, C3, and C4.
Students sometimes draw the beta anomer when asked for alpha (and vice versa). For D-sugars in a Haworth: alpha-OH is down at C1, beta-OH is up.
⚠️ The kinetic vs thermodynamic enolate distinction is a favourite exam question. Know which base gives which product and why.
⚠️ Acidity ranking of alpha-hydrogens between different carbonyl compounds (malonate vs beta-ketoester vs simple acid) appears regularly.
⚠️ Fischer-to-Haworth conversion is a common exam problem that looks hard but is entirely mechanical. Practise the rules until they are automatic.
⚠️ Know the difference between alpha and beta anomers in D-sugars. This single detail is worth easy marks.
True or False: LDA at −78 °C gives the thermodynamic enolate.
Answer: False. LDA gives the kinetic enolate (less substituted).
Fill in the blank: In a Fischer projection, groups on the right side go ________ in the Haworth projection.
Answer: Down.
True or False: The alpha anomer of a D-sugar has the OH at C1 pointing up in the Haworth projection.
Answer: False. Alpha has C1-OH pointing down.
Fill in the blank: A beta-ketoester has more acidic alpha-hydrogens than a simple ester because the alpha-H is flanked by ________.
Answer: Both a ketone carbonyl and an ester carbonyl (two electron-withdrawing groups).
True or False: NaOEt/HOEt deprotonation is irreversible.
Answer: False. It is reversible (equilibrating conditions), which is why the thermodynamic product predominates.
Q: A ketone with alpha-hydrogens on both sides is treated with LDA in THF at −78 °C. Which enolate forms?
A: The kinetic enolate, formed by deprotonation of the less substituted (less hindered) alpha carbon.
Q: The same ketone is treated with NaOEt in HOEt at room temperature. Which enolate forms?
A: The thermodynamic enolate, formed at the more substituted alpha carbon.
Q: Rank the following in order of decreasing acidity of their alpha-hydrogens: a malonate diester, a beta-ketoester, a simple carboxylic acid.
A: Beta-ketoester > malonate diester > carboxylic acid. The beta-ketoester's alpha-H is flanked by the strongest pair of electron-withdrawing groups.
Q: Convert D-glucose (Fischer projection) to the alpha-D-glucopyranose Haworth projection. Where does the OH at C1 point?
A: Down. The alpha anomer of a D-sugar has C1-OH pointing down in the Haworth. C2-OH is down (right in Fischer), C3-OH is up (left in Fischer), C4-OH is down (right in Fischer), and CH₂OH is up (D-sugar).
Q: What determines whether a sugar is classified as D or L?
A: The configuration of the highest-numbered chiral centre. If the OH is on the right in the Fischer projection, the sugar is D.
Alpha carbon chemistry connects forward to aldol reactions, Claisen condensations, and Michael additions (often covered in Organic Chemistry II), all of which use enolates as nucleophiles. Carbohydrate chemistry ties directly to hemiacetal formation (from the carbonyl addition unit) and will reappear in biochemistry when studying glycolysis, glycogen, and nucleic acid structure. The stereochemical skills used in Fischer-to-Haworth conversions also reinforce the chirality and projection material from early in the course.
Alpha carbon, alpha hydrogen, enolate, enol, tautomerism, LDA, lithium diisopropylamide, NaOEt, sodium ethoxide, kinetic enolate, thermodynamic enolate, kinetic vs thermodynamic control, pKa, acidity ranking, malonate, beta-ketoester, 1,3-dicarbonyl, Fischer projection, Haworth projection, pyranose, furanose, anomer, anomeric carbon, alpha anomer, beta anomer, D-sugar, L-sugar, D-glucose, glucopyranose, mutarotation, carbohydrate chemistry, sugar ring forms, Org Chem I Exam 3