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what is steric hindrance and hyper conjugation?


pls do answer my last 2 questions

7 years ago

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Answers : (4)

										

Steric hindrance or steric resistance occurs when the size of groups within a molecule prevents chemical reactions that are observed in related smaller molecules.


Although steric hindrance is sometimes a problem, it can also be a very useful tool, and is often exploited by chemists to change the reactivity pattern of a molecule by stopping unwanted side-reactions (steric protection). 


However, hyperconjugation has been suggested as an explanation for the preference of the staggered conformation of ethane because the steric hindrance of the small hydrogen atom is far too small.


Below hyperconjugation is fully explained.


 


7 years ago
										

Hyperconjugation in organic chemistry is the stabilizing interaction that results from the interaction of the electrons in a sigma bond (usually C-H or C-C) with an adjacent empty (or partially filled) non-bonding p-orbital or antibonding π orbital or filled π orbital to give an extended molecular orbital that increases the stability of the system. Only electrons in bonds that are β to the positively charged carbon can stabilize a carbocation by hyperconjugation.


 


4265-2088_6681_HyperconjugationInSubstitutedAlkenes.png


Steric hindrance or steric resistance occurs when the size of groups within a molecule prevents chemical reactions that are observed in related smaller molecules.


4265-803_6681_Regioselective_tritylation.png


 


Although steric hindrance is sometimes a problem, it can also be a very useful tool, and is often exploited by chemists to change the reactivity pattern of a molecule by stopping unwanted side-reactions (steric protection). Steric hindrance between adjacent groups can also restrict torsional bond angles. However, hyperconjugation has been suggested as an explanation for the preference of the staggered conformation of ethane because the steric hindrance of the small hydrogen atom is far too small.

7 years ago
										

Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape (conformation) and reactivity.


Steric hindrance  occurs when the size of groups within a molecule prevents chemical reactions that are observed in related smaller molecules. Although steric hindrance is sometimes a problem, it can also be a very useful tool, and is often exploited by chemists to change the reactivity pattern of a molecule by stopping unwanted side-reactions (steric protection). Steric hindrance between adjacent groups can also restrict torsional bond angles. However, hyperconjugation has been suggested as an explanation for the preference of the staggered conformation of ethane because the steric hindrance of the small hydrogen atom is very small....


The structure, properties, and reactivity of a molecule is dependent on straight forward bonding interactions including covalent bonds, ionic bonds, hydrogen bonds and lesser forms of bonding. This bonding supplies a basic molecular skeleton that is modified by repulsive forces. These repulsive forces include the steric interactions described above. Basic bonding and steric are at times insufficient to explain many structures, properties, and reactivity. Thus steric effects are often contrasted and complemented by electronic effects implying the influence of effects such as induction, conjunction, orbital symmetry, electrostatic interactions, and spin state. There are more esoteric electronic effects but these are among the most important when considering structure and chemical reactivity.


 


hyperconjugation in organic chemistry is the stabilizing interaction that results from the interaction of the electrons in a sigma bond (usually C-H or C-C) with an adjacent empty (or partially filled) non-bonding p-orbital or antibonding π orbital or filled π orbital to give an extended molecular orbital that increases the stability of the system. Only electrons in bonds that are β to the positively charged carbon can stabilize a carbocation by hyperconjugation.


Hyperconjugation has an effect on several chemical properties



  1. Bond length: Hyperconjugation is suggested as a key factor in shortening of sigma bonds (σ bonds) in such systems. For example, the single C-C bonds in 1,3-butadiene and methylacetylene are approximately 1.46 angstrom in length, much less than the value (1.54 angstrom) found in saturated hydrocarbons. This is due mainly to hyperconjugation that gives partial double-bond character of the bond.




  2. Dipole moments: The large increase in dipole moment of 1,1,1-trichloroethane as compared with chloroform can be attributed to hyperconjugated structures.





  3. The heat of formation of such molecules are greater than sum of their bond energies; and the heats of hydrogenation per double bond are less than the heat of hydrogenation of ethylene.

  4. Stability of carbocations:

(CH3)3C+ > (CH3)2CH+ > (CH3)CH2+ > CH3+
The C-C σ bond adjacent to the cation is free to rotate, and as it does so, the three C-H σ bonds of the methyl group in turn undergoes the stabilization interaction. The more adjacent C-H bonds are, the larger hyperconjugation stabilization is.
7 years ago
										

Steric hindrance is easily explained, and it appears as though it has been done.


 


Hyperconjugation involves the interaction of a sigma CH bonding orbital with an adjacent carbocation p-orbital, or a sigma CH bonding orbital with an adjacent CH sigma antibonding orbital. There are additional examples. Its best not to generalize this concept, but to focus on a concrete example.


 


Please read the article Newman Projection "True Lies" to understand how hyperconjugation works in the ethane molecule.

4 years ago

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