Carbonyl Organic mechanisms and reactions Flashcards
What does a carbonyl bond consist of?
A carbonyl bond (C=O) consists of one strong s-bond and a weaker pi-bond.
How many sites of reactivity do carbonyls have?
Three sites of reactivity
What are the three sites of reactivity for a carbonyl?
ẟ + carbon - electrophilic C - susceptible to attack from nucleophiles.
ẟ- oxygen - Lewis basic O - reacts with Lewis acids/Bronsted acids.
Acidic α-proton - weakly acidic α-proton - can be deprotonated to form C nucleophile.
What steps does both nucleophilic substitution and addition have?
- Nucleophilic attack on the carbonyl
2. Protonation of resulting anion or loss of leaving group
Why does the nucleophilic addition happen at the carbonyl carbon?
Nucleophiles donate pair of electrons into the π-antibonding orbital of the C=O.
To make a new bond between the carbonyl carbon and the nucleophile, you have to attack the antibonding orbital, and the lowest lying antibonding orbital is the π-antibonding orbital.
The reason they attack the carbonyl carbon is because it has the highest coefficient in the antibonding orbital. And the antibonding orbital, due to electrostatic repulsions, has a slightly shallower angle to the pi-bond.
Why does the nucleophile attack at 107 degrees during nucleophilic addition?
Net result, Nucleophile attacks at 107° (Bürgi-Dunitz angle).
The pi orbital is distorted towards the O (the electron density is pulled towards the O) - makes nucleophilic addition difficult because the charge is there it repels the nucleophile - so the antibonding orbital slightly rearranges its geometry to get around this electrostatic repulsion that occurs between the O and the nucleophile.
So instead of the nucleophile coming in at 90° , it actually comes at 107°, either above or below the C.
order these from most to least reactive: acid anhydride, amide, ester, acyl chloride
acyl chloride, acid anhydride, ester, amide,
Difference Between Inductive Effect and Resonance Effect?
Inductive effect is the effect caused by the induced electrical charges in atoms of a molecule. This charge induction occurs due to the differences in the electronegativity values of atoms.
The resonance effect of a molecule arises when there are double bonds in that molecule.
The main difference between inductive effect and resonance effect is that inductive effect describes the transmission of electrical charges between atoms in a molecule whereas resonance effect describes the transmission of electron pairs between atoms in a molecule.
How are carbonyls like aromatics are affected by resonance and inductive effects?
lone pairs on molecules, can donate into the pi system, which reduces the electropositivity at the carbonyl carbon atom, therefore making it less reactive.
So for acyl chlorides, the Cl pulls a lot out for its bond but it doesn’t put much back in from its lone pairs.
But for an amide molecule with the NH2 group, since N is less electronegative, it doesn’t pull as much out from the bond but it drops a lot back in from its lone pair, making the carbonyl carbon more electron rich.
Why are carboxylic acids not included in the carbonyl reactivity series?
When treated with a nucleophile, they get deprotonated and form acetate.
Why are aldehydes and ketones different from other carbonyls?
They contain alkyl groups on the carbonyl.
Do aldehydes and ketones undergo substitution at the carbonyl centre, and why?
Aldehydes and ketones do not undergo substitution at the carbonyl centre since they both have really poor leaving groups.
What is more reactive: aldehyde or ketone? Why?
Aldehyde is more reactive.
Inductive effects of alkyl groups surrounding carbonyl.
The aldehyde only has one alkyl group next to the carbonyl carbon so only one alkyl can donate electron density onto the carbonyl carbon whilst a ketone has 2, 2 alkyl groups is better than 1 so that makes the carbonyl carbon more electropositive.
Aldehydes are less hindered than ketones (a hydrogen atom is smaller than any other organic group).
The carbonyl carbon in aldehydes generally has more partial positive charge than in ketones due to the electron-donating nature of alkyl groups. Aldehydes only have one e- donor group while ketones have two.
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.
What is acid strength affected by?
Acid strength is affected by:
Strength on H-X bond.
Stability of conjugate base (most important for organic acids)
How do Carbonyls fit into acid base equilibrium?
Can delocalise the negative charge - form a resonance form and stick it on the O.
Why do carbonyl CHs have a lower pKa?
This is why carbonyl CHs have a much lower pKa because they can form a conjugate base.
Oxidation level definition?
Oxidation level: refers to the number of heteroatom bonds that are bonded to the atom in question.
What happens when you oxidise a primary alcohol?
Primary alcohols can be readily oxidised all the way to carboxylic acids.
What happens when you oxidise a secondary alcohol?
Secondary alcohols oxidised to ketones
What are two “Classic Methods’’ to achieve the oxidation of alcohols to carboxylic acids?
Jones Oxidation (Na2Cr2O7 , H2SO4 , H2O) Potassium Permanganate (H+ or HO- )
What is the problem with Jones oxidation?
The problem with Jones oxidation is that it uses stoichiometric amounts of Cr, so every time you do oxidation for primary alcohols you create2 eq of Cr (III) waste.
Chromium waste is particularly toxic and is hard to dispose of.
Three main types of selectivity?
Chemoselectivity
regioselectivity
stereoselectivity
What is Chemoselectivity?.
Chemoselectivity: which functional group will react.
What is Regioselectivity?
Regioselectivity: where it will react.
What is Stereoselectivity?
Stereoselectivity: how it will react.
What is the problem of using Jones or KMnO4 to oxidise a primary alcohol to an aldehyde?
Use of either Jones or KMnO4 will oxidise primary alcohols to carboxylic acids.
Problem is water - because you then form a hydrate which can be further oxidised.
What did E.J. Corey develop as an alternative to Jones reagent?
E.J. Corey developed an alternative to Jones reagent, since a CrO3 derivative was needed which doesn’t require aqueous conditions.
PCC is soluble in organic solvents..
Issues with PCC?
Not particularly green, creates a bi-product of Cr(III)
What is Ley Oxidation, and what does it use?
In 1987 S. V. Ley and co-workers introduced tetrapropylammonium perruthenate (TPAP) as a mild oxidation of primary / secondary alcohols to aldehydes or ketones.
What does TPAP utilise?
Utilises ruthenium in its +7 oxidation state.
Uses ruthenium as its oxidant
What is the co-oxidant used in Ley oxidation?
N-Methylmorpholine oxide (NMO)
Why is Ley oxidation relatively green?
Uses sub-stoichiometric amounts - so, for example, if you want to turn one mole of the alcohol to one mole of the aldehyde, you can use 0.1 moles or less of ruthenium, and as its catalyst its not decomposed and can be reused = greener alternative.
NMO allows you to recycle the ruthenium.
What is the Dess Martin Oxidation, and what does it use?
In 1983 D.B. Dess and J.C. Martin developed a hypervalent Iodine species soon to be known as the Dess-Martin periodinane (aka Dess Martin Reagent or DMP)
Is the Dess-Martin periodinane (DMP) reagent a mild stoichiometric oxidant?
Very mild stoichiometric oxidant.
Will stop at the aldehyde.
Creates stoichiometric amounts of DMP waste.
What is the Swern Oxidation, and what does it use?
In 1976 D. Swern described a sulfur based method for the oxidation of primary and secondary alcohols.
Utilises sulfur in its +4 oxidation state.
Multiple derivatives of the method.
Why is the Swern Oxidation done at low temperatures?
Done at a low temp, because reaction is fast and exothermic.
What is the Pinnick Oxidation, and what does it use?
In 1981 H.W. Pinnick described a mild method to oxidise aldehydes to carboxylic acids.
Sodium Chlorite (NaClO2 ) is the oxidant.
2-Methyl-2-butene is utilised as a radical scavenger.
2 examples of reducing agents?
metal hydrides
Lewis acid hydrides
Relationship between reactivity and selectivity in metal hydrides?
The more reactive the metal hydride, the less selective it is.
What are metal hydrides? Examples?
Metal hydrides are one of the most common reducing reagents you will come across.
Source of “H- ‘’ nucleophile.
Like carbonyl compounds, metal hydrides have different reactivities.
Eg. NaBH4
What are Lewis acid hydrides? Examples?
These are charge neutral Lewis Acids.
Only source of “H- ” when they form the Lewis acid-base complex.
Useful for reduction of electron rich carbonyl derivatives - such as ester and amides.
What is a Clemmensen reduction reaction and what does it use?
aldehyde/ketone -> alkane
uses: Zn(Hg), Concentrated HCl
What is a Wolff-Kishner reduction reaction and what does it use?
aldehyde/ketone -> alkane
N2H4.H2O, KOH, >180 degrees
What is cyanide an example of?
a carbon based nucleophile
What are nitriles easily converted into?
carboxylic acids
What does hydrolysis utilise?
Acid (H2SO4) and water
What are organometallic reagents? Most common type derived?
Organometallic reagents are compounds which contains carbon-metal bonds.
Alkyl groups can be added to carbonyls via organometallic reagents.
Li and Mg = most common derived type
How can organometallic reagents create new organometallic reagents?
They are very strong nucleophiles, and are very strong bases. pKa of conjugate acid >40.
Therefore can deprotonate acid C-H bonds to form new organometallic reagents.
Particularly Alkynes!
What is a Grignard reagent?
A Grignard reagent has a formula RMgX where X is a halogen, and R is an alkyl or aryl (based on a benzene ring) group.
Involves insertion of magnesium into carbon-halogen bond (“Oxidative Insertion”)
What is a Wittig Reaction?
In the early 1950’s G. Wittig and co co-workers described the use of phosphonium salts to convert aldehydes / ketones into alkenes.
More commonly known as the Wittig reaction.
What is the key reagent used in the Wittig reagent?
Utilises a “phosphorus ylid” as the key reagent. Which is prepared in situ from phosphonium salt.
How is phosphonium salt prepared?
Phosphonium salt is easily prepared from alkyl halide and phosphine.
What can substituted alkenes exist as?
Substituted alkenes can exist as either E/Z diastereoisomers.
What happens of you get an electron withdrawing group on the ylid?
If you have an electron withdrawing group on the phosphorus ylid, then you create what is known as a stabilised phosphorus ylid. If you have the stabilised phosphorus ylid then you get the E-alkene.
Are carboxylic acids a Bronsted acid or base and a Lewis acid or base?
Carboxylic Acids are Brønsted Acids and Lewis Bases.
Why are carboxylic acids a Bronsted acid?
Is a Bronsted acid due to the acidic proton.
The proton is acidic because when the molecule is being deprotonated and we form the conjugate base (carboxylate), we have a stable anion formed - stable due to negative charge being delocalised over 3 atoms.
Are carboxylic acids a strong or weak acid?
Strong acids, due to stability of conjugate base.
Have a really low pKa.
What are three ways of organic synthesis to produce a carboxylic acid?
- From primary alcohol: Jones Oxidation
- From aldehyde: Pinnick Oxidation
- From nitrile: Acid hydrolysis
What happens for a carboxylic acid to be converted to an acid chloride?
A Key functional group interconversion (FGI).
Replaces poor leaving group (-OH) with (-Cl).
Moves it much higher up the carbonyl reactivity series.
What reagent is used to convert a carboxylic acid to an acid chloride?
SOCl2
What happens for an acid chloride to be converted to an acid anhydride?
Replaces poor leaving group (-OH) with (-CO2R)
How can you form an acid anhydride from a carboxylic acid?
Start with carboxylic acid, then treat it with chlorinate reagents (SOCl2) to form an acid chloride. Then you can react your acid chloride with a carboxylate - carboxylates are nucleophilic due to the oxygen and the delocalisation effects of the negative charge into the pi-system, which makes the lone pairs more nucleophilic.
What happens when you react an acid chloride with organometallic reagents?
Acyl Chlorides will react with organometallics to give tertiary alcohols.
What type of reaction will acid anhydrides undergo?
hydrolysis