Table of Contents
What does a nucleophilic attack on a carbonyl by a hydride result in?
Nucleophilic attack of the hydride ion (H: – ) to the electrophilic carbon of the carbonyl group results in the reduction of aldehydes to primary alcohols and ketones to secondary alcohols.
What can attack a carbonyl?
As with aldehydes and ketones, protonation activates the carbonyl and the next step is attack by the nucleophile—in this case an alcohol.
What is a hydride attack?
The attack of hydride ions as nucleophiles on electrophilic carbon creates a new C–H bond, therefore the carbon atom undergoes a reduction (gain of bonds to hydrogen). Because of this, this type of reactions is commonly referred to as reductions, even though the mechanism is a nucleophilic addition.
What face does the hydride attack?
equatorial face
Hydrides can approach cyclohexanones from the axial or equatorial face of the ketone. It has been obvserved that increasingly bulky hydride reagents prefer to attack from the equatorial face of the carbonyl.
Why is the carbonyl carbon attacked by a nucleophile?
The attack of the nucleophile is through the electrophilic carbon atom of the target molecule. The electrophilic carbon atom becomes somewhat electron-deficient due to the pull of electrons toward the oxygen atoms. Because of this, the carbonyl carbon becomes highly susceptible to a nucleophilic attack.
Why is hydride a poor nucleophile?
Therefore, the hydride ion has a massive ionic radius and is easily polarized, making it an extremely soft base. Electrophilic carbon atoms in carbonyl groups are usually hard(er). Plus the 1s orbital of hydrogen is too small to effectively overlap with anything other than another hydrogen.
Can an alkene attack a carbonyl?
Since the carbonyl group is extremely polar across the carbon-oxygen double bond, this makes it susceptible to addition reactions like the ones that occur in the pi bond of Alkenes, especially by nucleophilic and electrophilic attack.
Which carbonyl compound is more susceptible to nucleophilic attack?
The most susceptible to nucleophilic attack at the carbonyl group is acetyl chloride (CH3COCl). In general, acid chlorides are most susceptible to nucleophilic attack at the carbonyl group as chloride ion is a very good leaving group.
What is a hydride reagent?
A reagent that, in a reaction, acts as the hydride ion would if it were nucleophilic is called a hydride reagent or hydride equivalent. The most common hydride reagents are the reducing agents sodium borohydride (NaBH4) and lithium aluminumhydride (LiAlH4).
Which atom is attacked by the hydride ion source H − in the first step?
carbon atom
In the first stage, there is a nucleophilic attack by the hydride ion on the slightly positive carbon atom.
What makes carbonyl compounds very reactive?
Factors that place more positive charge on the carbonyl (electron withdrawing groups nearby) make the carbonyl more positive and more reactive. Factors that place additional electron density on the carbonyl (electron donors nearby) make the carbonyl less reactive.
What is the order of reactivity of carbonyl groups?
In nucleophilic addition reactions, the reactivity of carbonyl compounds follows order –
- A. H2C=O>R2C=ORCHO>
- B. H2C=O>RCHO>ArCHO>R2CO>Ar2.
- C. Ar2C=O>R2C=O>ArCHO>RCHO>
- D. ArCHO>Ar2C=O>RCHO>R2C=O>
Are hydrides always nucleophilic?
For inorganic chemists, hydrides refer to compounds and ions in which hydrogen is covalently attached to a less electronegative element. In such cases, the H centre has nucleophilic character, which contrasts with the protic character of acids. The hydride anion is very rarely observed.
Is hydride a base or nucleophile?
Sodium hydride (NaH): The sodium salt of the conjugate base of H2; Na+ :H-. A strong base (H2 pKa 35). Despite its high basicity, NaH is not nucleophilic. Use of NaH to generate an alkyne anion by deprotonation of the terminal hydrogen of propyne.
Can a pi bond attack a carbonyl?
Can an alkene attack a Carbocation?
Electrophilic addition is probably the most common reaction of alkenes. Consider the electrophilic addition of H-Br to but-2-ene: The alkene abstracts a proton from the HBr, and a carbocation and bromide ion are generated. The bromide ion quickly attacks the cationic center and yields the final product.
Why are carbonyl functional groups susceptible to nucleophilic attacks?
Which compound would react more rapidly with a nucleophile?
While the presence of electron releasing groups such as R or OR increases the electron density, thus deactivate the nucleus toward nucleophilic attack. NO2 group activate the ring more than Cl toward nucleophilic attack hence react readily with a nucleophile.
What are hydrides in chemistry?
A hydride is the anion of hydrogen, H−, or, more commonly, it is a compounds in which one or more hydrogen centers have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, the hydrogen atom is bonded to a more electropositive element or group.