Hyperconjugation Definition

The presence of a suitable attacking reagent can cause electron displacement in an organic molecule. The bond is polarised as a result of this type of electron displacement. The electromeric effect and hyperconjugation are two effects that involve electron displacement. 

Electromeric effect: 

The electromeric effect is a transient reaction that occurs when an attacking reagent comes into contact with an organic molecule with multiple bonds (a double or triple bond). On the demand of an attacking reagent, the complete transfer of a shared pair of -electrons to one of the atoms connected by numerous bonds occurs in this effect. When the attacking reagent is withdrawn from the reaction region, the effect stops. The electromeric effect is primarily divided into two types. 

Positive electromeric effect (+E effect): 

The transfer of π-electrons from many bonds to the atom with which the reagent is bonded is known as the positive electromeric effect. 

Negative electromeric effect (-E effect): 

The transfer of π-electrons from multiple bonds to the atom with which the reagent does not connect is known as the negative electromeric effect. 

Hyperconjugation: 

The hyperconjugation effect is a permanent effect in which σ electrons of an alkyl group’s C-H bond are directly connected to an unsaturated system atom or an atom with an unshared p orbital. 

One of the three C-H bonds of the methyl group can align in the plane of the empty p orbital, and the electrons comprising the C-H bond in this plane can then be delocalized into the empty p orbital, as seen in the diagram above.

The hyperconjugation additionally stabilises the carbocation by assisting in the distribution of positive charges. As a result, we may say that the more alkyl groups connected to a positively charged carbon atom, the more hyperconjugation interaction and carbonation stabilisation there is. On the basis of hyperconjugation, the relative stability is given as, 

Effect on chemical properties: 

Several qualities are affected by hyperconjugation: 

1. Bond length: Hyperconjugation has been proposed as a crucial element in sigma bond (σ bond) shortening. The single C–C bonds in 1,3-butadiene and Propyne, for example, are about 1.46 angstrom long, substantially shorter than the 1.54 angstrom observed in saturated hydrocarbons. This can be explained in the case of butadiene by regular conjugation of the two alkenyl components. Hyperconjugation between the alkyl and alkynyl portions of Propyne, on the other hand. 

2. Dipole moments: Hyperconjugative structures are responsible for the substantial increase in dipole moment of 1,1,1-trichloroethane when compared to chloroform. 

3. Heat of formation: The heats of hydrogenation per double bond are fewer than the heats of hydrogenation of ethylene, and the heats of formation of molecules with hyperconjugation are more than the total of their bond energies. 

4. Stability of carbocations: The three C–H σ bonds of the methyl group(s) connected to the carbocation can experience the stabilising interaction, but depending on the conformation of the carbon–carbon bond, only one of them can be fully aligned with the empty p-orbital. 

(CH3)3C+  >  (CH3)2CH+   >  (CH3)CH2+  >  CH3+  

The two mismatched C–H bonds have a weaker donation. Because of the greater number of neighbouring C–H bonds, the larger hyperconjugation stabilisation occurs when there are more adjacent methyl groups.  

Causes: 

Hyperconjugation is a stabilising reaction that occurs when electrons in a σ-bond engage with either a nearby partially full or vacant p-orbital or a π-orbital to generate a longer molecular orbital. 

Applications of hyperconjugation: 

The anomeric effect, the gauche effect, the rotational barrier of ethane, the beta-silicon effect, the vibrational frequency of exocyclic carbonyl groups, and the relative stability of substituted carbocations and substituted carbon centred radicals, as well as the thermodynamic Zaitsev’s rule for alkene stability, can all be explained using hyperconjugation. More controversially, quantum mechanical modelling suggests that hyperconjugation, rather than the conventional textbook concept of steric hindrance, is a better explanation for the preference for the staggered conformation. 

Negative Hyperconjugation: 

Negative hyperconjugation is the donation of electron density from a filled – or p-orbital to a nearby σ*-orbital in organic chemistry. The molecule or transition state can be stabilised by this event, which is a type of resonance. It also lengthens the -bond by increasing electron density in the antibonding orbital. 

Negative hyperconjugation is uncommon, while it occurs frequently when the σ*-orbital is placed on particular C–F or C–O bonds, and it does not occur to a significant degree with conventional C–H bonds.

The electron density moves in the opposite direction (from π– or p-orbital to empty σ*-orbital) in negative hyperconjugation than in the more typical hyperconjugation (from a lone pair of electrons to an empty p-orbital). 

β-position: 

The number of atoms distant from the metal is indicated by the Greek letters. The alpha position refers to the initial atom bonded to the metal. An alpha hydrogen is a hydrogen on that atom. The beta position refers to the next atom in the chain. The gamma position is the third atom in the chain. 1,2-elimination or beta-elimination can occur when hydrogen is linked to the beta location. 

• A double bond is formed as a result of elimination.

• Between the alpha and beta positions, a double bond occurs. 

Because a carbonyl compound already has an alpha and a beta position, this nomenclature can be misleading. The carbonyl carbon is in position, and the carbon adjacent to the carbonyl is in position. This Greek alphabet system is a broad technique of marking positions that is utilised in a variety of circumstances; you must be able to determine which context is appropriate. If elimination is taking place, the phrase “position” could refer to one of two things. If enolate formation is taking place, the term “position” has a different meaning. 

Conclusion: 

The delocalization of electrons from a single link between hydrogen and another atom in the molecule is known as hyperconjugation. The electrons in the bond have been delocalized. Furthermore, the hydrogen atom has no link with the other atom. As a result, the broken connection might be held responsible for the conjugation’s feasibility. Hyperconjugation is also known as no bond resonance since there is no bond between the hydrogen and the other atom.