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1-aminocyclopropane-1-carboxylate is the carboxyl-deprotonated form of 1-aminocyclopropane-1-carboxylic acid: a three-carbon ring bears an amino group and a carboxylate group on the same ring carbon. Its specified anionic form has formula ${\mathrm{C}}_4{\mathrm{H}}_6\mathrm{N}{\mathrm{O}}_2^{-}$. The familiar biochemical abbreviation ACC is also often used for the acid without specifying protonation.^[ref]

It is an intermediate between S-adenosylmethionine and Ethylene in plants. The 1979 apple experiments showed that labelled ACC became Ethylene in air and bypassed an inhibitor that blocked conversion from methionine. A supplied downstream intermediate restoring product formation helps locate the blocked reaction upstream.^[ref]

An alpha-hydroxy acid is a carboxylic acid with a hydroxyl group on the carbon immediately adjacent to the carboxyl group. That neighboring carbon is the alpha carbon; in simple systematic numbering it is carbon 2 because the carboxyl carbon is carbon 1. The hydroxyl oxygen within the carboxyl group itself is not the additional hydroxyl that defines this class.^[ref]

For a common saturated monocarboxylic example, the local structure can be writtenwith $R,R^{\prime }$ representing hydrogen or carbon-containing substituents. Glycolic acid, $\mathrm{HOC}{\mathrm{H}}_2\mathrm{COOH}$, has two hydrogens at that carbon. Lactic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$, has hydrogen and methyl groups there and is also called 2-hydroxypropanoic acid.^[ref][ref]

Position distinguishes the class. The structure $\mathrm{HOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{COOH}$ has its additional hydroxyl group at carbon 3 and is a beta-hydroxy acid, even though it has the same molecular formula as lactic acid. A molecular formula therefore does not determine whether a compound is an alpha-hydroxy acid; the connectivity is essential.

The alpha carbon can also be a stereogenic centre. In lactic acid it is bonded to four different groups: hydrogen, hydroxyl, methyl and carboxyl. Its two enantiomers share the alpha-hydroxy classification but differ in three-dimensional arrangement. Glycolic acid does not have that stereogenic centre because two substituents are identical hydrogens.

Both functional groups participate in the chemistry. Loss of the carboxyl proton gives a carboxylate, such as lactate from lactic acid, while the additional hydroxyl remains part of the structure. Distinguishing group identity, position, protonation and stereochemistry makes the class useful in organic and biological chemistry without reducing it to a list of commercial uses.

Aspartate denotes an anionic form of aspartic acid. A common biochemical example is the L-configured monoanion, with two carboxylate groups and one positively charged amino group. Its formula is ${\mathrm{C}}_4{\mathrm{H}}_6\mathrm{N}{\mathrm{O}}_4^{-}$ and its net charge is $-1$.^[ref]

The charge count isHaving both positive and negative groups therefore does not imply a zero total charge. Compared with neutral aspartic acid, formation of this conjugate base removes a proton; it does not change the four-carbon connectivity or, by itself, reverse the L/D stereochemistry.

An illustrative acid–base comparison keeps these questions separate. For one protonation step in a dilute solution, the Henderson-Hasselbalch equation gives $[\mathrm{base}]/[\mathrm{acid}]\simeq 10^{\mathrm{pH}-\mathrm{p}{K}_a}$. One pH unit above that step's $\mathrm{p}{K}_a$ gives a ratio of about ten. This is a symbolic illustration, not a numerical assignment of the molecule's several dissociation constants. Specifying ionic form, pH and stereochemistry is more informative than treating every use of the name aspartate as one context-free structure.

A beta-hydroxy acid is a carboxylic acid with a hydroxyl group on the beta carbon: the carbon two carbon–carbon bonds away from the carboxyl group carbon. For a simple chain numbered from the carboxyl carbon as carbon 1, the beta carbon is carbon 3. The hydroxyl within the carboxyl group itself does not establish the additional hydroxy substitution.

The small example 3-hydroxypropionic acid has structureStarting at the carboxyl carbon, the adjacent $\mathrm{C}{\mathrm{H}}_2$ is alpha and the terminal $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ is beta. The primary ChEBI record gives the systematic name 3-hydroxypropanoic acid, formula ${\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3$ and its classification as a 3-hydroxy monocarboxylic acid.^[ref]

An alpha-hydroxy acid instead places the additional hydroxyl group one carbon closer to the carboxyl group. Thus lactic acid, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{OH})\mathrm{COOH}$, and 3-hydroxypropionic acid, $\mathrm{HOC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{COOH}$, have the same molecular formula but different atom connectivity. They are constitutional isomers; counting oxygen atoms or measuring molecular mass alone cannot distinguish them. The distinction is structural, not a statement about acidity or biological activity.^[ref]

The two functional groups also describe different possible transformations. Removing the carboxyl proton forms a carboxylate while preserving the beta-hydroxy carbon skeleton. Converting the hydroxyl group to a carbonyl changes the hydroxy-acid functionality. Which transformation occurs depends on reagents and conditions; the class name alone supplies no reaction rate or treatment claim. Tracking positions relative to the carboxyl group makes such structural changes easier to compare.

A carboxylate is the conjugate base of a carboxylic acid; an acetate ion is one example.

A carboxylate bound through one or both oxygen atoms. It can be monodentate, chelating or bridging; its charge is $-1$ regardless of whether one or two oxygen donors bind.

A carboxyl group is the functional group $-\mathrm{C}(=\mathrm{O})\mathrm{OH}$: one carbon has a double bond to oxygen and a single bond to a hydroxyl group. It gives a carboxylic acid its characteristic acid functionality. In acetic acid, the group is attached to a methyl group, giving $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}$.^[ref]

Loss of the acidic hydrogen produces a carboxylate, the conjugate base:The pH influences the balance between these forms. A structural drawing of the neutral acid therefore need not show the predominant charge state in every solution. The carboxyl group is a part of the molecule, while acetic acid names the entire compound.

A Gla domain is a protein domain containing several gamma-carboxyglutamate residues. These are glutamate residues changed by a Vitamin K-dependent post-translational modification: an additional carboxyl group gives the modified residue extra capacity to bind calcium ions. In proteins such as prothrombin, the resulting calcium ion interactions help organize the domain's protein folding and its association with a phospholipid membrane. The residue chemistry, the folded domain and its membrane interaction are related but distinct levels of explanation.

The unusual residue was identified experimentally in 1974. J. Stenflo and colleagues found that a prothrombin fragment's electrophoretic mobility could not be explained by its ordinary amino acid composition. Nuclear magnetic resonance and mass spectrometry identified gamma-carboxyglutamic acid. The extra chemical group helped explain the fragment's calcium ion-binding behaviour.^[ref]

Later, M. Soriano-Garcia and colleagues used X-ray crystallography to resolve a calcium-bound prothrombin fragment at $2.2\text{A˚}$ resolution. Much of the Gla domain was disordered in the calcium-free crystal, whereas the calcium-bound structure was well defined and contained seven bound calcium ions. Their coordination by carboxylate groups and water connected residue chemistry to a three-dimensional structure. Seven is the count for that reported structure, rather than a universal definition of every Gla domain.^[ref]

Four equivalents of base remove the four carboxylic-acid protons from two ligand molecules, giving two dianionic pyridine-2,5-dicarboxylate linkers per Pt/Mn pair. The ring nitrogen and neighbouring 2-carboxylate supply a five-membered N,O chelate. A consistent chain has Pt(II), N${}_2$O${}_2$, with square planar molecular geometry, and Mn(II), O${}_6$, with octahedral molecular geometry.

At each Pt, two ligands bind through their ring N and one O of the adjacent 2-carboxylate; their remote 5-carboxylates point toward successive Mn sites. At each Mn, a remote carboxylate from each of two neighbouring Pt-containing units binds through both O atoms. Two coordinated water molecules complete Mn's six-coordinate sphere. The repeating connectivity, with each bracket denoting a single organic linker, isEach Pt has a second N,O chelate extending in the opposite chain direction; each Mn similarly receives two carboxylate chelates. The original donor-connectivity diagram makes those counts explicit.

Pt(II) is $5d^8$ and strongly favours square planar molecular geometry. Its affinity for the pyridyl N donor and the chelate effect select the mixed N,O pocket. Mn(II) is high-spin $3d^5$, is harder, and favours the carboxylate and water O donors; six-coordinate octahedral molecular geometry is natural in water. Charge balance per repeat is $+2+2+2(-2)=0$. A suitable repeat formulation is $[\mathrm{PtMn}(\mathrm{L}{)}_2({\mathrm{H}}_2\mathrm{O}{)}_2{]}_{\mathrm{n}}$. This is a justified proposed coordination chain, not a unique crystal structure established from stoichiometry alone; water content, carboxylate bridging modes and actual porosity require structural characterization.

The first base equivalent removes O–H; the second removes the alpha C–H. The first equivalent of lithium diisopropylamide converts the carboxylic acid to its lithium carboxylate. A further equivalent removes the remaining hydrogen on the carbon between the two allyl groups, giving a carboxylic acid dianion:The second negative charge has enolate-like delocalization into the carboxyl group; lithium coordination and aggregation are not represented by this simple charge-bookkeeping structure. The carbon nucleophile attacks methyl iodide with C–I bond cleavage, forming the new C–Me bond. acid workup reprotonates the carboxylate, giving $F=\mathrm{H}{\mathrm{O}}_2\mathrm{C}-\mathrm{C}(\mathrm{Me})(\mathrm{L}{)}_2$. The two equivalents are therefore chemically necessary for the stated carbon alkylation rather than simply an arbitrary excess. A single equivalent is consumed predominantly by the much more acidic carboxylic proton. The product has formula ${\mathrm{C}}_9{\mathrm{H}}_{14}{\mathrm{O}}_2$ and still has two identical allyl groups, so this methylation does not create a stereocenter.

hydroxide attacks an ester carbonyl to form a tetrahedral intermediate. Collapse expels ethoxide, and acid–base transfer gives the carboxylate and ethanol. Repeating this nucleophilic acyl substitution at the other ester produces the malonate dianion; acid workup gives $\mathrm{H}{\mathrm{O}}_2\mathrm{C}-\mathrm{C}(\mathrm{L}{)}_2-\mathrm{C}{\mathrm{O}}_2\mathrm{H}$, with $L=\mathrm{C}{\mathrm{H}}_2\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$ as defined above.

On heating, one carboxylic acid group is lost through a six-membered cyclic proton-transfer/bond-cleavage arrangement. A carbonyl oxygen of the retained group accepts the proton from the departing carboxyl OH as its C–C bond breaks and carbon dioxide forms; electron redistribution gives the enol-like form of the retained acid. proton transfer/tautomerization then producesThe net decarboxylation is ${\mathrm{C}}_9{\mathrm{H}}_{12}{\mathrm{O}}_4\to {\mathrm{C}}_8{\mathrm{H}}_{12}{\mathrm{O}}_2+\mathrm{C}{\mathrm{O}}_2$. The central carbon gains a hydrogen, both allyl substituents survive, and only one carboxyl group is removed. It would be incorrect to draw hydrolysis directly giving the final acid without accounting for this carbon loss. The favourable cyclic pathway is characteristic of malonic acid decarboxylation.

A glutamic acid residue supplies a carboxyl group whose protonation state can change with the active site environment. The following five schematic contributions are possible in a hypothetical enzyme; they are alternatives and overlapping physical effects, rather than five independently established reactions of this enzyme. In the diagrams $E$ is the protein tether, and $C6$ is the reacting purine carbon.

general acid catalysis: $\mathrm{E}-\mathrm{C}{\mathrm{O}}_2\mathrm{H}\cdots N1\to \mathrm{E}-\mathrm{C}{\mathrm{O}}_2^{-}+\mathrm{H}-\mathrm{N}{1}^{+}$. Donation of the carboxyl proton activates the amidine-like C6 center for water attack. The same acid could instead assist the amino leaving group: $\mathrm{E}-\mathrm{C}{\mathrm{O}}_2\mathrm{H}+-\mathrm{N}{\mathrm{H}}_2\to \mathrm{E}-\mathrm{C}{\mathrm{O}}_2^{-}+-\mathrm{N}{\mathrm{H}}_3^{+}$. These are two locations for general acid catalysis, rather than two different categories.

general base catalysis: $\mathrm{E}-\mathrm{C}{\mathrm{O}}_2^{-}\cdots \mathrm{H}-\mathrm{OH}\to \mathrm{E}-\mathrm{C}{\mathrm{O}}_2\mathrm{H}+\mathrm{O}{\mathrm{H}}^{-}$, coupled to oxygen attack on C6. Accepting a proton makes water a more effective nucleophile. This does not require an appreciable pool of free hydroxide in bulk solution.

covalent catalysis: a glutamate oxygen can in principle attack activated C6 to form $C6-\mathrm{O}-\mathrm{C}(\mathrm{O})-\mathrm{E}$. After addition/elimination replaces the amino group, water must hydrolyze that transient enzyme–substrate bond to release product and regenerate $\mathrm{E}-\mathrm{C}{\mathrm{O}}_{2}H/C{\mathrm{O}}_2^{-}$. Such a pathway is a chemically distinct possibility, not established by the given structure.

electrostatic catalysis: the carboxylate can stabilize a developing positive charge, for example in a protonated purine or amino leaving group; a protonated carboxyl can also donate a hydrogen bond to an oxygen developing negative charge. This contributes only if the reacting configuration is stabilized relative to the bound starting state. A carboxylate placed next to an oxyanion without compensating geometry would instead be unfavorable.

proximity and orientation catalysis: donor and acceptor hydrogen bonds can hold the purine and attacking water in a productive arrangement, schematically $\mathrm{E}-\mathrm{C}{\mathrm{O}}_2\mathrm{H}\cdots N1-C6\cdots \mathrm{O}{\mathrm{H}}_2$. Binding reduces unproductive orientations and can distort the substrate toward a reactive geometry. This last contribution is sometimes described through substrate binding or strain; a binding interaction is not automatically catalytic unless it improves access to the transition state.

For hydrogen-bond acceptance, the three strongest groups are the terminal carboxylate, the Cys–CO–N(Pro) tertiary-amide carbonyl, and the Pro–CO–NH(Tyr) secondary-amide carbonyl, in that qualitative order. Negative charge makes carboxylate a particularly strong acceptor. Both amide oxygens accept strongly; greater nitrogen alkyl donation generally makes a tertiary amide oxygen more basic than the comparable secondary-amide oxygen. This last ordering is a functional-group estimate, not a precise calculation of a folded tripeptide's site parameters.

For donation, the order is terminal ammonium $>$ tyrosine phenolic OH $>$ the peptide NH between Pro and Tyr. The thiol is a weaker donor. Protonated ammonium has no available lone pair to accept; the proline amide nitrogen has no N–H bond to donate, and amide nitrogen lone pairs are delocalized into carbonyl groups. The carboxylate has no O–H donor in the drawn protonation state.

Aqueous base selectively converts the methyl ester to a carboxylate, and brief acidification gives the corresponding monoacid while retaining the tert-Butyl ester. At the methyl-ester end, a hydroxide lone pair attacks the acyl carbon and the carbonyl pi pair moves to oxygen. The tetrahedral intermediate collapses as the oxygen anion reforms C=O and the acyl C–OMe bond breaks to methoxide. proton transfer then gives the carboxylate and methanol. carboxylate formation makes the net saponification effectively irreversible under the basic conditions; protonation protonates that carboxylate.

The tert-Butyl ester reacts more slowly under these conditions because its bulky alkoxy group hinders acyl attack, and tert-butoxide is a stronger base and poorer leaving group than methoxide. The selectivity is kinetic, not an assertion that tert-butyl esters can never undergo base hydrolysis. The work-up should be sufficiently mild and brief to avoid the separate acid-labile pathway.

Anhydrous trifluoroacetic acid instead removes the tert-butyl group, leaving the methyl ester. protonation activates the ester; in a useful reactive protonation form the alkoxy oxygen bears the proton. The O–C(tert-butyl) bond then cleaves with its electron pair going to oxygen, producing the carboxylic acid and a tert-butyl-cation-like fragment. Loss of a methyl proton gives 2-methylpropene and regenerates the acid catalyst. Carbonyl-oxygen protonation and charge delocalization can precede cleavage; drawing alkoxy-oxygen protonation is a transparent way of tracking the bonds broken. The key distinction is alkyl–oxygen cleavage here versus acyl–oxygen cleavage in the base reaction. Methyl-cation formation is strongly unfavourable, and without water ordinary acid-catalysed methyl-ester hydrolysis is not available. No water is required to account for the tert-butyl deprotection or its alkene by-product.

Build the phenethyl grignard reagent, use one portion to make a phenethyl-substituted ester, and add two further equivalents to that ester. Write $R=\mathrm{PhC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2$; the target is ${\mathrm{R}}_3\mathrm{COH}$, so all three carbon substituents must be phenethyl, not benzyl.

hydroboration followed by oxidation of styrene gives $\mathrm{PhC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$: use $\mathrm{B}{\mathrm{H}}_3$ in a suitable ether, followed by alkaline hydrogen peroxide. The Anti-Markovnikov addition regiochemistry is essential; acid-catalysed Markovnikov addition of water would give the wrong alcohol. Convert the primary alcohol to $\mathrm{PhC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{Br}$ with phosphorus tribromide, then form $\mathrm{RMgBr}$ with magnesium in dry ether. The organomagnesium reagent must be prepared and used without a protic solvent.

carboxylate one portion with carbon dioxide and acidify to obtain $\mathrm{RC}{\mathrm{O}}_2\mathrm{H}$, then esterify with ethanol under acid catalysis to give $\mathrm{RC}{\mathrm{O}}_2\mathrm{Et}$. Treat the isolated ester with at least two equivalents of another portion of $\mathrm{RMgBr}$ and then perform an aqueous acidic work-up:The first addition to the ester gives a ketone after ethoxide elimination; the second addition gives the tertiary alkoxide. Work-up supplies its OH proton. The extra central carbon comes from carbon dioxide, and each original styrene contributes one intact eight-carbon phenethyl unit. The resulting carbon count is $1+3\times 8=25$, matching the target skeleton.

The dianion of pyridine-2,5-dicarboxylic acid, useful as a bridging ligand because its N,O donor pair and its more remote carboxylate can bind different metal centres.

A pyridine ring bearing carboxylic acid substituents at positions 2 and 5. Removing both acidic protons gives a dianion with a neighbouring N,O chelation site and a remote carboxylate site.

Saponification is hydrolysis of an ester under basic conditions. The conductometric saponification assay follows ethyl acetate consumption by hydroxide, producing ethanol and an acetate ion; its equal-starting-concentration case provides a practical second-order reaction example.

Saponification is ester hydrolysis promoted by a chemical base, with net consumption of hydroxide to form a carboxylate and an alcohol. For a carboxylic acid-derived ester, the net chemical equation isThe hydroxide is a reactant, rather than merely a regenerated catalyst.^[ref]

For ethyl acetate, the balanced chemical equation becomesBoth sides contain four carbon chemical atoms, nine hydrogen chemical atoms, three oxygen chemical atoms, and net electric charge $-1$. A sodium ion supplied by sodium hydroxide is a spectator ion; the net ionic equation omits it without losing electric charge conservation.

In the usual reaction mechanism, hydroxide adds to the carbonyl group, making a reaction intermediate. Departure of the alcohol's conjugate base gives a carboxylic acid; transfer of a proton then gives the carboxylate and the alcohol. Subsequent addition of a chemical acid gives the carboxylic acid by protonation:This separate acidification changes the reaction product's protonation state.^[ref]

The conductometric saponification assay makes hydroxide consumption observable. A university procedure equilibrates separate ethyl acetate and sodium hydroxide stocks in constant-temperature baths, then mixes them in a rinsed conductivity cell. Two platinum electrodes and an alternating-voltage Wheatstone bridge supply an electrical resistance readout. A matched sodium hydroxide–water dilution checks the initial signal; potassium chloride calibrates the cell constant. Replacing hydroxide with less mobile acetate ions lowers electrical conductance, supporting an inference about changing concentration. The manual supplies a procedure rather than measured outcomes. Temperature drift or additional ions can change the signal; it cannot alone identify the full reaction mechanism.^[ref]

A triglyceride contains three ester linkages joining glycerol to fatty acid-derived acyl groups. Complete saponification therefore consumes three hydroxide equivalents per triglyceride, yielding one glycerol and three fatty acid carboxylates.^[ref]

Writing the three hydrocarbon groups separately allows their compositions to differ:This is the preceding ester chemical equation applied three times: each linkage supplies one carboxylate, and each formerly esterified oxygen on the glycerol scaffold gains one hydrogen. The two sides retain the same three ${\mathrm{R}}_i$ groups and have net electric charge $-3$.

Soap consists of sodium or potassium salts of long-chain fatty acids. Its carboxylate head is hydrophilic, while its long hydrocarbon tail is hydrophobic. This makes it an amphiphile and a surfactant: the tails can associate with grease while the heads remain exposed to water, helping disperse grease for rinsing. An acetate ion is also a carboxylate, but lacks the long hydrophobic tail that gives soap this cleaning role.^[ref]

Consider a hypothetical closed, well mixed chemical solution initially containing $20\mathrm{mmol}$ of ethyl acetate, $12\mathrm{mmol}$ of hydroxide, and no acetate ions or ethanol. One millimole is $10^{-3}$ mole. Assume complete conversion as far as the one-to-one chemical equation allows, with no competing chemical reactions. This is an ideal stoichiometry calculation, not a measured yield.

Let the extent of reaction be $\xi$, expressed in millimoles. The four values of amount of substance, in the order ethyl acetate, hydroxide, acetate ion, ethanol, areThe upper bound follows because both reactant quantities must be nonnegative. Thus hydroxide is the limiting reagent, and the ideal endpoint $\xi =12$ leavesThe carboxylate formed equals the hydroxide consumed, while unreacted ester remains. Adding more hydroxide raises the stoichiometry bound; merely waiting longer does not change that bound. To convert these values of amount of substance into concentrations, a volume is also required. The endpoint does not specify the reaction rate or time needed to approach it.