LDA, or lithium diisopropylamide, is a strong base used in organic chemistry to pull a proton off a molecule, generating a reactive intermediate that can then participate in further reactions. Its conjugate acid (diisopropylamine) has a pKa of 35.7 in THF, making LDA strong enough to deprotonate nearly any carbon-hydrogen bond next to a carbonyl group. What makes it special is that it almost exclusively acts as a base, not as a nucleophile, so it removes protons without attacking the carbonyl carbon itself.
Why LDA Acts as a Base, Not a Nucleophile
LDA’s nitrogen atom sits between two bulky isopropyl groups. These branching carbon chains create a steric shield around the nitrogen, physically blocking it from approaching and attacking electrophilic carbons like the carbon of a carbonyl group. A smaller base like sodium hydride or hydroxide might add to the carbonyl instead of (or in addition to) pulling off a neighboring proton. LDA’s bulk makes that addition pathway extremely unlikely for most substrates, so the reaction cleanly produces the deprotonated product you want.
That said, LDA is not completely incapable of acting as a nucleophile. In reactions with highly electrophilic aromatic rings, such as fluorine-substituted pyridines, LDA can displace a leaving group through direct substitution. These cases are uncommon and generally considered side reactions in most synthetic contexts.
Enolate Formation: The Most Common Role
The reaction chemists use LDA for most often is creating enolates from carbonyl compounds like ketones, esters, and amides. An enolate forms when LDA removes a hydrogen from the carbon directly next to the carbonyl (the alpha carbon). This generates a negatively charged carbon that is stabilized by the neighboring carbonyl group. The enolate can then react with an electrophile, such as an alkyl halide in an alkylation reaction, or with another carbonyl in an aldol reaction.
Because LDA is such a strong base, it deprotonates the substrate completely and irreversibly before any electrophile is added. This is different from weaker bases like sodium hydroxide, which establish an equilibrium where only a small fraction of the starting material exists as the enolate at any given time. With LDA, you get full, clean conversion to the enolate first, then add the electrophile in a separate step. This two-step approach gives you much more control over the outcome.
Kinetic vs. Thermodynamic Enolates
When a ketone has alpha hydrogens on both sides of the carbonyl, LDA preferentially removes the less sterically hindered proton. This is the proton on the less substituted side, and the resulting enolate is called the kinetic enolate. For example, when LDA deprotonates 2-heptanone, only about 7% of the product is the more substituted (thermodynamic) enolate. The selectivity comes from LDA’s bulk: it approaches the less crowded hydrogen because that’s physically easier.
This selectivity depends on temperature. Reactions are typically run at -78 °C (the temperature of a dry ice/acetone bath) to lock in the kinetic product. At warmer temperatures, the enolate can equilibrate to the thermodynamic form, which is more stable but not always the one you want. The combination of a bulky base and a very cold reaction is what gives chemists precise control over which enolate forms.
There are exceptions. When the more substituted side is stabilized by something like an adjacent phenyl group, even LDA can favor the thermodynamic enolate. Deprotonation of phenyl acetone with LDA produces the thermodynamic isomer as 66% of the product, because the conjugation with the aromatic ring makes that enolate significantly more stable.
Beyond Enolates: Other Reactions LDA Mediates
While enolate chemistry is the headline use, LDA participates in several other reaction types:
- Lithiation of imines: LDA can deprotonate the carbon adjacent to a C=N bond, creating a nitrogen-stabilized carbanion useful for building carbon-carbon bonds.
- 1,4-addition to unsaturated esters: In the presence of certain co-solvents, LDA adds to the beta position of an alpha,beta-unsaturated ester, producing beta-amino esters in yields around 76%.
- Epoxide opening: LDA can abstract a proton from an epoxide, generating a carbenoid intermediate. In one reported case, this pathway gave an alcohol product in 80% isolated yield through a transannular C-H insertion.
- Ortholithiation and Fries rearrangement: LDA removes a proton directly from an aromatic ring adjacent to a directing group like a carbamate, triggering a subsequent rearrangement.
How LDA Is Prepared
LDA is made by mixing two reagents: diisopropylamine and n-butyllithium, usually in THF as the solvent. The butyllithium deprotonates the amine to generate LDA in solution. Both reagents need to be handled carefully. Butyllithium is typically sold as a solution in hexanes and must be titrated before use to confirm its concentration. Diisopropylamine is distilled fresh before the reaction. The THF solvent is also dried rigorously, since any water would destroy the base.
Pre-made commercial solutions of LDA are also available. These are more convenient and notably non-pyrophoric, unlike butyllithium itself, which can ignite spontaneously on contact with air or moisture.
The Role of THF and Temperature
THF is not just an inert container for LDA. The oxygen atom in THF coordinates to the lithium, stabilizing the reagent and influencing its structure. In THF solution, LDA exists primarily as dimers (two LDA units paired together), and many reactions require these dimers to break apart into monomers before the actual deprotonation can occur.
At -78 °C, this dimer-to-monomer conversion can actually become the slowest step in the entire reaction, meaning the overall rate depends on how fast LDA breaks apart rather than how fast it reacts with the substrate. Interestingly, even trace amounts of lithium chloride (as little as 1 part per million) can catalyze this breakup, dramatically changing reaction rates. This sensitivity to tiny impurities explains why LDA reactions can sometimes behave unpredictably between different batches of reagents or glassware.
LDA Compared to Other Strong Bases
LDA is not the only option for generating enolates, but it fills a specific niche. LiHMDS (lithium hexamethyldisilazide) is another non-nucleophilic base that is commercially available and easier to handle. It is slightly weaker than LDA, which can be an advantage when reversible deprotonation is desired, giving cleaner results in some alkylation reactions. Sodium-based versions of these amide bases (NaHMDS, NaDA) produce sodium enolates instead of lithium enolates, which behave differently in subsequent reactions like aldol additions.
Sodium hydride is a much simpler base that can also form enolates, but it lacks the steric bulk of LDA and does not offer the same kinetic selectivity with unsymmetrical ketones. For reactions where you specifically need the less substituted enolate, LDA at -78 °C remains the standard choice.