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Is An Aldehyde Or Ketone More Acidic?

Acidity Of Alpha Hydrogens & Keto-enol Tautomerism

Acidity Of Alpha Hydrogens & Keto-enol Tautomerism

Alkyl hydrogen atoms bonded to a carbon atom in a a (alpha) position relative to a carbonyl group display unusual acidity. While the pKa values for alkyl C-H bonds is typically on the order of 40-50, pKa values for these alpha hydrogens is more on the order of 19-20. This can most easily be explained by resonance stabilization of the product carbanion, as illustrated in the diagram below. In the presence of a proton source, the product can either revert back into the starting ketone or aldehyde or can form a new product, the enol. The equilibrium reaction between the ketone or aldehyde and the enol form is commonly referred to as "keto-enol tautomerism". The ketone or aldehyde is generally strongly favored in this reaction. Because carbonyl groups are sp2 hybridized the carbon and oxygen both have unhybridized p orbitals which can overlap to form the C=O bond. The presence of these overlapping p orbitals gives hydrogens (Hydrogens on carbons adjacent to carbonyls) special properties. In particular, hydrogens are weakly acidic because the conjugate base, called an enolate, is stabilized though conjugation with the orbitals of the carbonyl. The effect of the carbonyl is seen when comparing the pKa for the hydrogens of aldehydes (~16-18), ketones (~19-21), and esters (~23-25) to the pKa of an alkane (~50). Of the two resonance structures of the enolate ion the one which places the negative charge on the oxygen is the most stable. This is because the negative change will be better stabilized by the greater electronegativity of the oxygen. Keto-enol Tautomerism Because of the acidity of α hydrogens carbonyls undergo keto-enol tautomerism. Tautomers are rapidly interconverted constitutional isomers, usually distinguished by a different bonding location for a labile hydrogen Continue reading >>

Oxidation Of Aldehydes And Ketones

Oxidation Of Aldehydes And Ketones

This page looks at ways of distinguishing between aldehydes and ketones using oxidising agents such as acidified potassium dichromate(VI) solution, Tollens' reagent, Fehling's solution and Benedict's solution. Background Why do aldehydes and ketones behave differently? You will remember that the difference between an aldehyde and a ketone is the presence of a hydrogen atom attached to the carbon-oxygen double bond in the aldehyde. Ketones don't have that hydrogen. The presence of that hydrogen atom makes aldehydes very easy to oxidise. Or, put another way, they are strong reducing agents. Note: If you aren't sure about oxidation and reduction, it would be a good idea to follow this link to another part of the site before you go on. Alternatively, come back to this link if you feel you need help later on in this page. Use the BACK button (or HISTORY file or GO menu if you get seriously waylaid) on your browser to return to this page. Because ketones don't have that particular hydrogen atom, they are resistant to oxidation. Only very strong oxidising agents like potassium manganate(VII) solution (potassium permanganate solution) oxidise ketones - and they do it in a destructive way, breaking carbon-carbon bonds. Provided you avoid using these powerful oxidising agents, you can easily tell the difference between an aldehyde and a ketone. Aldehydes are easily oxidised by all sorts of different oxidising agents: ketones aren't. You will find details of these reactions further down the page. What is formed when aldehydes are oxidised? It depends on whether the reaction is done under acidic or alkaline conditions. Under acidic conditions, the aldehyde is oxidised to a carboxylic acid. Under alkaline conditions, this couldn't form because it would react with the alkali. A salt Continue reading >>

Why Are Aldehydes And Ketones Neutral And Not Acidic/basic?

Why Are Aldehydes And Ketones Neutral And Not Acidic/basic?

This question is quite general, and as other answers have pointed out, depends on what you mean by acidic/basic. The other answers have already covered the Lewis bit, so I will focus a bit more on the Bronsted-Lowry and Arrhenius definitions. In water, almost all aldehydes and ketones do not dissociate to give the H+ or OH- ion, which is the Arrhenius definition. However, the alpha position of acetylacetone is considered acidic by the Bronsted-Lowry definition, and can be deprotonated by strong bases to give the conjugated enone after tautomerism. This is due to the stability of the conjugate base. Continue reading >>

Chemistry Of Enolates And Enols – Acidity Of Alpha-hydrogens

Chemistry Of Enolates And Enols – Acidity Of Alpha-hydrogens

In the presence of carbonyl functional group, the alpha-hydrogens of a molecule exhibit acidity i.e. in the presence of a base they can be abstracted very easily to yield a carbanion. The acidity of the α-hydrogen of carbonyl compounds depends on the stability of the carbanion formed (which is the conjugate base in this case). If the carbanion is more stable, the alpha-hydrogen is more acidic. The carbanion can be stabilized either with resonance – i.e. the carbanion lone pair to the oxygen of the carbonyl to form the stabilized enolate, or by inductive effect – if electron withdrawing groups are directly attached to the alpha-carbon. Stabilization by inductive effect - if electron withdrawing groups are directly attached to the alpha-carbon. Note – Acidities of the alpha-hydrogen is measured in pKa. Lower pKa value of the hydrogen, more acidic it is. Comparison of Acidities of Alpha – Hydrogens Note – The pKa values are given assuming the R’ and R” groups are alkyl (mostly methyl group) and are an approximate value. 1] α-Hydrogens of Ketones vs Aldehydes The alpha-hydrogens of ketones (pKa = 20) are less acidic as compared to aldehydes (pKa = 17). This is because the alkyl group R” of ketones pushes electrons via inductive effect on to the alpha-carbon. This would increase the electron density at the alpha-carbon to slightly destabilize the formation of the conjugate base – carbanion. 2] α-Hydrogens of Ketones vs Esters The alpha-hydrogen of ketones (pKa = 20) is more acidic as compared to the alpha-hydrogens of esters (pKa = 25). The reason for this is that the ester functional group has free lone pairs on the oxygen which can participate in resonance with carbonyl group. This resonance competes with the resonance of the stabilization of the enola Continue reading >>

Reactions Of Aldehydes And Ketones

Reactions Of Aldehydes And Ketones

Aldehydes and ketones undergo a variety of reactions that lead to many different products. The most common reactions are nucleophilic addition reactions, which lead to the formation of alcohols, alkenes, diols, cyanohydrins (RCH(OH)C&tbond;N), and imines R 2C&dbond;NR), to mention a few representative examples. The main reactions of the carbonyl group are nucleophilic additions to the carbon‐oxygen double bond. As shown below, this addition consists of adding a nucleophile and a hydrogen across the carbon‐oxygen double bond. Due to differences in electronegativities, the carbonyl group is polarized. The carbon atom has a partial positive charge, and the oxygen atom has a partially negative charge. Aldehydes are usually more reactive toward nucleophilic substitutions than ketones because of both steric and electronic effects. In aldehydes, the relatively small hydrogen atom is attached to one side of the carbonyl group, while a larger R group is affixed to the other side. In ketones, however, R groups are attached to both sides of the carbonyl group. Thus, steric hindrance is less in aldehydes than in ketones. Electronically, aldehydes have only one R group to supply electrons toward the partially positive carbonyl carbon, while ketones have two electron‐supplying groups attached to the carbonyl carbon. The greater amount of electrons being supplied to the carbonyl carbon, the less the partial positive charge on this atom and the weaker it will become as a nucleus. The addition of water to an aldehyde results in the formation of a hydrate. The formation of a hydrate proceeds via a nucleophilic addition mechanism. 1. Water, acting as a nucleophile, is attracted to the partially positive carbon of the carbonyl group, generating an oxonium ion. Acetal formation reacti Continue reading >>

Carbonyl Condensations And Alpha Substitution

Carbonyl Condensations And Alpha Substitution

1. Structure and Formation of Enols and Enolate ions 1a. Enols and Tautomerization 1b. Enolate Ions 2. Halogenation and Alkylation of Enolates and Enols (a-substitution) 2a. Haloform Reaction 2b. Hell-Volhard-Zelinsky Reaction 2c. Alkylation of aldehydes and ketones using LDA 3. Aldol Condensation 3a. Mixed Aldol Reaction 3b. Aldol Cyclization 4. Claisen Ester Condensation 4a. The Dieckman Cyclization- Claisen Cyclization Process 4b. Mixed Claisen Reaction 5. The Acetoacetic Ester Synthesis 5a. Decarboxylation Mechanism 6. The Malonate Ester Synthesis 6a. Variations on the Malonate ester synthesis 6b. Knoevenagel Condensation 7. Retrosynthetic Analysis 7a. Synthetic Strategy for Aldol Condensations (Synthesis of b-Hydroxy Aldehydes and Ketones and a,b-Unsaturated Aldehydes and Ketones) 7b. Synthetic Strategy for Claisen Condensations (Synthesis of b-keto Esters) 7c. Synthetic Strategy for Malonate Ester Synthesis (Acetic Acid Synthesis) 8. Conjugate Additions- The Michael Reaction 9. The Robinson Annulation 10. Active Methylene Compounds 11. The Mannich Reaction 1. Structure and Formation of Enols and Enolate ions 1a. Enols and Tautomerization Tautomers – special type of constitutional isomers that are easily interconverted: Keto-enol tautomerization is an equilibrium process. The keto form predominates. (The keto form refers to the aldehyde (R=H) and the ketone, i.e., the carbon-oxygen double bond form.) A hydrogen atom on an a carbon is said to be enolizable. Without any acid or base present, two molecules may undergo tautomerization: Tautomerization is catalyzed by acids … … and catalyzed by bases: 1b. Enolate Ions Enolate ions are produced under basic conditions: Be sure to distinguish between enols and enolates … Enolates are ions- resonance shows them to b Continue reading >>

Introduction To Ch-acidic Compounds

Introduction To Ch-acidic Compounds

The Acidity of Carbonyl Compounds Aldehydes and ketones have remarkably low pKa values that range between 15 and 20. Thus, they may act as a Brønsted acid in an acid-base reaction with a strong base. The acidic hydrogen is the hydrogen that is bound to the carbon adjacent to the carbonyl carbon. Due to the only minor difference in electronegativity between hydrogen and carbon, C-H bonds in alkanes are hardly polarized at all. Thus, hydrogens of alkanes are in fact not acidic. The pKa values of alkanes amount to only approximately 50. The acidic hydrogen of carbonyl compounds is known as α hydrogen, as it is connected with the carbonyl compound's α carbon that is directly bound to the carbonyl carbon. The carbonyl compounds' relatively high acidity may be explained by the resonance stabilization of the conjugate base by the carbonyl group, or, in other words, through the stabilization of the anion formed by deprotonation. This anion is called an enolate anion. The negative charge is mainly distributed among the α carbon and the carbonyl oxygen, by resonance, which leads to the stabilization of the otherwise highly energized carbanion. Stability of enolate anions Both the bonding and non-bonding π orbitals of the enolate are occupied, while the antibonding π orbital remains free. The distribution of the negative charge and the nucleophilic qualities of the enolate anion are mostly represented through the occupied non-bonding π orbital, as it is the enolate's HOMO. The illustration of the non-bonding π orbital is indicative of the location of the non-bonding π electrons, which are at the α carbon and the carbonyl oxygen. Thus, the α carbon and the carbonyl oxygen are the nucleophilic positions of enolate anions. The smaller extension of the non-bonding π orbita Continue reading >>

Acidity Of

Acidity Of

a-Hydrogens In the following table, the acidity of the H for various enolate systems and other closely related systems are given. You should be able to justify the trends in this data ! Why are the protons adjacent to carbonyl groups acidic ? As we have advocated before, look at the stabilisation of the conjugate base. Notice the proximity of the adjacent p system, and hence the possibility for RESONANCE stabilisation by delocalisation of the negative charge to the more electronegative oxygen atom. The more effective the resonance stabilisation of the negative charge, the more stable the conjugate base is and therefore the more acidic the parent system. Let's compare pKa of the common systems: aldehyde pKa = 17, ketone pKa = 19 and an ester pKa = 25, and try to justify the trend. The difference between the 3 systems is in the nature of the group attached to the common carbonyl. The aldehyde has a hydrogen, the ketone an alkyl- group and the ester an alkoxy- group. H atoms are regarded as having no electronic effect : they don't withdraw or donate electrons. Alkyl groups are weakly electron donating, they tend to destabilise anions (you should recall that they stabilise carbocations). This is because they will be "pushing" electrons towards a negative system which is unfavourable electrostatically. Hence, the anion of a ketone, where there are extra alkyl groups is less stable than that of an aldehyde, and so, a ketone is less acidic. In the ester, there is also a resonance donation from the alkoxy group towards the carbonyl that competes with the stabilisation of the enolate charge. This makes the ester enolate less stable than those of aldehydes and ketones so esters are even less acidic. The most important reactions of ester enolates are the Claisen and Dieckmann cond Continue reading >>

Organic Chemistry/ketones And Aldehydes

Organic Chemistry/ketones And Aldehydes

Aldehydes () and ketones () are both carbonyl compounds. They are organic compounds in which the carbonyl carbon is connected to C or H atoms on either side. An aldehyde has one or both vacancies of the carbonyl carbon satisfied by a H atom, while a ketone has both its vacancies satisfied by carbon. 3 Preparing Aldehydes and Ketones Ketones are named by replacing the -e in the alkane name with -one. The carbon chain is numbered so that the ketone carbon, called the carbonyl group, gets the lowest number. For example, would be named 2-butanone because the root structure is butane and the ketone group is on the number two carbon. Alternatively, functional class nomenclature of ketones is also recognized by IUPAC, which is done by naming the substituents attached to the carbonyl group in alphabetical order, ending with the word ketone. The above example of 2-butanone can also be named ethyl methyl ketone using this method. If two ketone groups are on the same structure, the ending -dione would be added to the alkane name, such as heptane-2,5-dione. Aldehydes replace the -e ending of an alkane with -al for an aldehyde. Since an aldehyde is always at the carbon that is numbered one, a number designation is not needed. For example, the aldehyde of pentane would simply be pentanal. The -CH=O group of aldehydes is known as a formyl group. When a formyl group is attached to a ring, the ring name is followed by the suffix "carbaldehyde". For example, a hexane ring with a formyl group is named cyclohexanecarbaldehyde. Aldehyde and ketone polarity is characterized by the high dipole moments of their carbonyl group, which makes them rather polar molecules. They are more polar than alkenes and ethers, though because they lack hydrogen, they cannot participate in hydrogen bonding like Continue reading >>

Carbonyl Alpha-substitution Reactions

Carbonyl Alpha-substitution Reactions

Alpha-substitution reactions occur at the position next to the carbonyl group, the α-position, and involve the substitution of an α hydrogen atom by an electrophile, E, through either an enol or enolate ion intermediate.[1] Alpha substitution scheme Reaction mechanism[edit] Because their double bonds are electron rich, enols behave as nucleophiles and react with electrophiles in much the same way that alkenes do. But because of resonance electron donation of a lonepair of electron s on the neighboring oxygen, enols are more electron- rich and correspondingly more reactive than alkenes. Notice in the following electrostatic potential map of ethenol (H2C=CHOH) how there is a substantial amount of electron density on the α carbon. When an alkene reacts with an electrophile, such as HCl, initial addition of H+ gives an intermediate cation and subsequent reaction with Cl− yields an addition product. When an enol reacts with an electrophile, however, only the initial addition step is the same. Instead of reacting with CI− to give an addition product, the intermediate cation loses the OH− proton to give an α-substituted carbonyl compound.[1]:845 Alpha-halogenation of aldehydes and ketones[edit] A particularly common α-substitution reaction in the laboratory is the halogenation of aldehydes and ketones at their α positions by reaction Cl2, Br2 or I2 in acidic solution. Bromine in acetic acid solvent is often used. Remarkably, ketone halogenation also occurs in biological systems, particularly in marine alga, where dibromoacetaldehyde, bromoacetone, 1, l,l -tribromoacetone, and other related compounds have been found. The halogenation is a typical α-substitution reaction that proceeds by acid catalyzed formation of an enol intermediate.[1]:846 Acidity of alpha-hydro Continue reading >>

Acidity Of

Acidity Of

Acidity of a-Hydrogens In the following table, the acidity of the H for various enolate systems and other closely related systems are given. You should be able to justify the trends in this data ! Why are the protons adjacent to carbonyl groups acidic ? As we have advocated before, look at the stabilization of the conjugate base. Notice the proximity of the adjacent p system, and hence the possibility for RESONANCE stabilization by delocalisation of the negative charge to the more electronegative oxygen atom. The more effective the resonance stabilization of the negative charge, the more stable the conjugate base is and therefore the more acidic the parent system. Let's compare pKa of the common systems: aldehyde pKa = 17, ketone pKa = 19 and an ester pKa = 25, and try to justify the trend. The difference between the 3 systems is in the nature of the group attached to the common carbonyl. The aldehyde has a hydrogen, the ketone an alkyl- group and the ester an alkoxy- group. H atoms are regarded as having no electronic effect : they don't withdraw or donate electrons. Alkyl groups are weakly electron donating, they tend to destabilize anions (you should recall that they stabilize carbocations). This is because they will be "pushing" electrons towards a negative system which is unfavourable electrostatically. Hence, the anion of a ketone, where there are extra alkyl groups is less stable than that of an aldehyde, and so, a ketone is less acidic. In the ester, there is also a resonance donation from the alkoxy group towards the carbonyl that competes with the stabilization of the enolate charge. This makes the ester enolate less stable than those of aldehydes and ketones so esters are even less acidic. The most important reactions of ester enolates are the Claisen and Die Continue reading >>

Carbonyls: 10 Key Concepts (part 2)

Carbonyls: 10 Key Concepts (part 2)

If you’re on the typical college cycle, chances are you’re taking Org II right now. As you are by now well aware [more aware than you wish you were, I can hear some of you say] one of the main focii of Org II is on the many different facets of carbonyl chemistry. Following on the heels of the previous post, here are five further important points to keep in mind when studying the chemistry of these species. [Note – these will be included in the second carbonyl summary sheet, currently in preparation]. Without further ado… 6. Carbonyls make adjacent alkyl groups more acidic. How much more acidic? Consider ethane. For all practical purposes, ethane is inert to base: the pKa of its hydrogens is 50. But when you exchange one of the protons of ethane for a carbonyl group (say, -COOCH3) something phenomenal happens. The acidity changes by a factor of 10 ^25. That is an incredible number to wrap your head around. The difference in chemical reactivity between different species can be incredibly, mind-bogglingly vast. We are not talking about the difference in quarterbacking ability between Peyton Manning and Dan Marino here, or even the comparative basketball skill set of LeBron James and Verne Troyer. The differences in reactivity* are truly cosmic: like comparing the width of your armspan to the length of the Milky Way galaxy. What’s going on here? Simply put, the carbonyl π system provides a “sink” for the carbanion to donate electron density, setting up a sharing of negative charge between the α-carbon and the carbonyl oxygen. The key structural feature is not so much the resonance, although that is a factor – [the lower pKa of propene (42) is a good example of resonance stabilization] The reason why the carbonyl stabilizes the carbanion so much is that the Continue reading >>

Chapter 19: Enols And Enolates Of Carbonyl Compounds And Their Reactions

Chapter 19: Enols And Enolates Of Carbonyl Compounds And Their Reactions

We have seen that the carbonyl group of aldehydes and ketones is highly reactive, and that additions to this functionality are common. In the present chapter we will see that not only is the carbonyl functionality reactive per se, but that it also activates nearby carbon-hydrogen bonds (specifically alpha hydrogens) to undergo a variety of substitution reactions. ENOLS. qEnols are isomers of aldehydes or ketones in which one alpha hydrogen has been removed and replaced on the oxygen atom of the carbonyl group. The resulting molecule has both a C=C (-ene) and an –OH (-ol) group, so it is referred to as an enol. Strictly speaking, to be an enol the –OH and the C=C must be directly attached to one another, i.e., in conjugation with each other, as shown below. qWe shall see that enols can be formed either by acid or base catalysis and that, once formed, they are highly reactive toward electrophiles (i.e., they are pretty strong nucleophiles). We shall also see that not all carbonyl compounds can form enols, but only those which have hydrogens of the alpha type. The carbon of an aldehyde or the two carbons of a ketone which are directly attached to the carbonyl carbon are designated as alpha carbons, and any hydrogens directly attached to these carbon atoms are termed alpha hydrogens. There can be more than one type of alpha hydrogen, or there may be no alpha hydrogens in a given carbonyl compound. qWe should not be surprised at all that in the equilibrium between a carbonyl compound and its corresponding enol, the equilibrium lies well to the carbonyl side, i.e., usually only small amounts of enol are present in the equilibrium. The preference for the carbonyl form over the enol form derives from the well-known circumstance that the C=O function is imuch more stable tha Continue reading >>

18.1: Acidity Of Aldehydes And Ketones: Enolate Ions

18.1: Acidity Of Aldehydes And Ketones: Enolate Ions

For alkylation reactions of enolate anions to be useful, these intermediates must be generated in high concentration in the absence of other strong nucleophiles and bases. The aqueous base conditions used for the aldol condensation are not suitable because the enolate anions of simple carbonyl compounds are formed in very low concentration, and hydroxide or alkoxide bases induce competing SN2 and E2 reactions of alkyl halides. It is necessary, therefore, to achieve complete conversion of aldehyde or ketone reactants to their enolate conjugate bases by treatment with a very strong base (pKa > 25) in a non-hydroxylic solvent before any alkyl halides are added to the reaction system. Some bases that have been used for enolate anion formation are: NaH (sodium hydride, pKa > 45), NaNH2 (sodium amide, pKa = 34), and LiN[CH(CH3)2]2 (lithium diisopropylamide, LDA, pKa 36). Ether solvents like tetrahydrofuran (THF) are commonly used for enolate anion formation. With the exception of sodium hydride and sodium amide, most of these bases are soluble in THF. Certain other strong bases, such as alkyl lithium and Grignard reagents, cannot be used to make enolate anions because they rapidly and irreversibly add to carbonyl groups. Nevertheless, these very strong bases are useful in making soluble amide bases. In the preparation of lithium diisopropylamide (LDA), for example, the only other product is the gaseous alkane butane. Because of its solubility in THF, LDA is a widely used base for enolate anion formation. In this application, one equivalent of diisopropylamine is produced along with the lithium enolate, but this normally does not interfere with the enolate reactions and is easily removed from the products by washing with aqueous acid. Although the reaction of carbonyl compound Continue reading >>

Functional Group Names, Properties, And Reactions

Functional Group Names, Properties, And Reactions

Functional Groups Functional groups refer to specific atoms bonded in a certain arrangement that give a compound certain physical and chemical properties. Learning Objectives Define the term “functional group” as it applies to organic molecules Key Takeaways Functional groups are often used to “functionalize” a compound, affording it different physical and chemical properties than it would have in its original form. Functional groups will undergo the same type of reactions regardless of the compound of which they are a part; however, the presence of certain functional groups within close proximity can limit reactivity. Functional groups can be used to distinguish similar compounds from each other. functional group: A specific grouping of elements that is characteristic of a class of compounds, and determines some properties and reactions of that class. functionalization: Addition of specific functional groups to afford the compound new, desirable properties. The Role of Functional Groups In organic chemistry, a functional group is a specific group of atoms or bonds within a compound that is responsible for the characteristic chemical reactions of that compound. The same functional group will behave in a similar fashion, by undergoing similar reactions, regardless of the compound of which it is a part. Functional groups also play an important part in organic compound nomenclature; combining the names of the functional groups with the names of the parent alkanes provides a way to distinguish compounds. The atoms of a functional group are linked together and to the rest of the compound by covalent bonds. The first carbon atom that attaches to the functional group is referred to as the alpha carbon; the second, the beta carbon; the third, the gamma carbon, etc. Simi Continue reading >>

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