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Is An Aldehyde Or Ketone More Acidic?

Oxygen Containing Compounds - Aldehydes And Ketones

Oxygen Containing Compounds - Aldehydes And Ketones

Description nomenclature Aldehyde suffix: -al, -aldehyde. Ketone prefix: keto-, oxo-. Ketone suffix: -one, ketone. physical properties C=O bond is polar, with the carbon partially positive and oxygen partially negative. Dipole-dipole interactions give these molecules higher boiling points than their corresponding alkanes, but not as high as the corresponding alcohols or carboxylic acids. infrared absorption of C=O bond: 1700 cm-1 Important reactions nucleophilic addition reactions at C=O bond acetal, hemiacetal Aldehydes and ketones react with 1 equivalent of alcohols to make hemiacetals. Aldehydes and ketones react with 2 equivalent of alcohols to make acetals. Hemiketal and ketal are the same as acetals except the starting compound must be a ketone and not an aldehyde. This is an old naming scheme that is no longer used. imine, enamine Secondary amine + aldehyde or ketone = enamine. reactions at adjacent positions haloform reactions Ketones + halogen = halogenation of the alpha position (carbon adjacent to the C=O group). Methyl ketone + halogen = haloform + carboxylate. Trihalogenated methyl = good leaving group. aldol condensation 2 acetaldehyde -> aldo. Works for carbonyl compounds with an acidic alpha proton. oxidation: aldehydes oxidize to carboxylic acids. Ketones do not oxidize further. 1,3-dicarbonyls: internal H-bonding Also referred to as active methylene compounds. Tautomerism causes one of the carbonyls to switch to its enol form, which contains an -OH group that hydrogen bonds with the other carbonyl C=O group on the same molecule. This is called intramolecular (internal) hydrogen bonding. keto-enol tautomerism Enol form is the one with the alcohol. Keto form is the one with the ketone. Keto form is more stable, it is the predominant form. organometallic 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 >>

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 >>

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 >>

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 >>

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 >>

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 >>

Carbonyl Functional Groups

Carbonyl Functional Groups

Chapter 15 - Carbonyl Compounds usually subdivided into two families: aldehydes and ketones both R groups are C or H carboxyl derivatives X is some electronegative element (halogen, O, N, S, others) The Carbonyl Group - Structure and Properties polar C=O double bond O is nucleophilic reacts with acids & electrophiles C is electrophilic reacts with Lewis bases and nucleophiles alpha-C-H position is acidic (pKa ~ 20) Aldehyde Nomenclature IUPAC: alkanal -al suffix, with carbonyl assumed #1 in parent chain -carbaldehyde suffix (if aldehyde can't be part of the parent) e.g., cyclohexanecarbaldehyde common nomenclature: formaldehyde acetaldehyde propionaldehyde butyraldehyde benzaldehyde Ketone Nomenclature IUPAC: alkanone -one suffix, with number -oxo- prefix if necessary e.g., 4-oxopentanal common nomenclature: dialkyl ketone acetone acetophenone benzophenone acyl- as prefix e.g., acetyl or benzoyl, as in acetylcyclopropane Precedence Order of Functional Groups decides which gets to be the parent compound (suffix) lower functional groups must be named as substituents (prefixes) carboxylic acid aldehyde ketone alcohol amine thiol Characteristic Properties polar H-bond acceptors, but not H-bond donors Mass spec shows alpha cleavage (stable acylium ion RCO+) or McLafferty rearrangement (split between alpha and beta carbons) NMR shows aldehyde C-H at 9-10 ppm, C=O at 180-210 ppm IR shows strong C=O stretch at 1600-1800 cm-1 exact position depends on ring strain, conjugation UV shows weak n -> pi* at ~ 300 nm and pi -> pi* at ~ 200 nm exact position and strength of the absorption depends on conjugation Nucleophilic Addition to Carbonyls the major reaction of carbonyl compounds General Trends: aldehydes more reactive than ketones (due to steric hindrance with ketones) aromatic c Continue reading >>

Why Is The Alpha Hydrogen On An Ester Less Acidic Than The Alpha Hydrogens Of Aldehydes Or Ketones? Shoudln't The The Two Carbonyl Oxygens Of The Ester Stabalize The Conjugate Base Making It Very Acidic When Protonated?

Why Is The Alpha Hydrogen On An Ester Less Acidic Than The Alpha Hydrogens Of Aldehydes Or Ketones? Shoudln't The The Two Carbonyl Oxygens Of The Ester Stabalize The Conjugate Base Making It Very Acidic When Protonated?

Here you can see the deprotonation of acetone and acetaldehyde, respectively. For acetone and acetaldehyde specifically, the explanation is simply that the substituent on acetone is larger than on acetaldehyde (CH3 vs. H). The larger the substituents attached, the more electron-electron repulsion the deprotonated other carbon has with the carbonyl oxygen (larger substituent, more steric strain from orbital crowding, more repulsion). Their resonance structures are identical aside from those two substituents as the difference. Similarly, here you can see the enolate of methyl acetate: For methyl acetate vs. acetone specifically, besides the substituent size, knowing how it has two competing resonance structures (deprotonated carbon with carbonyl vs. the alkoxyl oxygen with the carbonyl), if we look at the oxygen-oxygen resonance structure, the alkoxyl oxygen stabilizes the carbonyl. But that leaves the δ− carbon less likely to stabilize with the carbonyl oxygen due to similar partial charges, lessening the amount of possible resonance stabilization. Continue reading >>

Enolate Formation From Ketones

Enolate Formation From Ketones

Voiceover: In order to see how to form enolate anions, and in this video we're just gonna look in more detail how to form enolate anions from ketones. And so the ketone we have here is acetone. To find our alpha carbon, we just look at the carbon next to our carbonyl carbon, so this could be an alpha carbon, and this could be an alpha carbon. Each one of those alpha carbons has three alpha protons, and so there's a total of six. I'm just gonna draw one in here, and this is the one that we're going to show being deprotonated here. So, the base that's going to deprotonate acetone, we're gonna use LDA, which is Lithium Diisopropyl Amide And, I could go ahead and draw in the Lithium here, so Li Plus, and then we see the two isopropyl groups like that, a negative one charge on our nitrogen. So this is a very strong base, it's also very bulky and sterically hindered. So you can think about a lone pair of electrons in the nitrogen, taking that proton, leaving these electrons behind on this carbon, so we can go ahead and draw the conjugate base here. We would have electrons on this carbon now, that's a carbanion, so let me go ahead and show those electrons, these electrons in here magenta, are gonna come off onto this carbon. And this carbon is a [carbanae] because remember there's also two other hydrogens attached to it. So that's what gives it a negative one formal charge here. We can draw our resonance structure, we can show these electrons in magenta moving in here, these electrons coming off onto our oxygen, so for our resonance structure we would show the negative charge is now on our oxygen, this would be a negative one formal charge like that now. So the electrons in magenta moved into here to form our double bond, and then we can show the electrons in here in the blue 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 >>

Reactions At The Α-carbon

Reactions At The Α-carbon

Many aldehydes and ketones undergo substitution reactions at an alpha carbon, as shown in the following diagram (alpha-carbon atoms are colored blue). These reactions are acid or base catalyzed, but in the case of halogenation the reaction generates an acid as one of the products, and is therefore autocatalytic. If the alpha-carbon is a chiral center, as in the second example, the products of halogenation and isotopic exchange are racemic. Indeed, treatment of this ketone reactant with acid or base alone serves to racemize it. Not all carbonyl compounds exhibit these characteristics, the third ketone being an example. Two important conclusions may be drawn from these examples. First, these substitutions are limited to carbon atoms alpha to the carbonyl group. Cyclohexanone (the first ketone) has two alpha-carbons and four potential substitutions (the alpha-hydrogens). Depending on the reaction conditions, one or all four of these hydrogens may be substituted, but none of the remaining six hydrogens on the ring react. The second ketone confirms this fact, only the alpha-carbon undergoing substitution, despite the presence of many other sites. Second, the substitutions are limited to hydrogen atoms. This is demonstrated convincingly by the third ketone, which is structurally similar to the second but has no alpha-hydrogen. 1. Mechanism of Electrophilic α-Substitution Kinetic studies of these reactions provide additional information. The rates of halogenation and isotope exchange are essentially the same (assuming similar catalysts and concentrations), and are identical to the rate of racemization for those reactants having chiral alpha-carbon units. At low to moderate halogen concentrations, the rate of halogen substitution is proportional (i.e. first order) to aldehyde Continue reading >>

Like This Study Set?

Like This Study Set?

Sort How can you form one product with a crossed aldol addition reaction using LDA? (18.12) If both aldehydes have α-hydrogens. LDA used to remove the α-hydrogens that creates the enolate ion. (LDA is a strong base- all carbonyl compound converted to an enolate so none of the carbonyl compound will be left for the enolate ion to react with an aldol addition) Aldol addition cannot occur until the second carbonyl compound is added slowly (minimize chance that aldehyde w/ α-hydrogen forming an enolate ion and reacting with it parent compound) 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 >>

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 >>

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