Steric effects
Geometric aspects of ions and molecules affecting their shape and reactivity
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape (conformation) and reactivity of ions and molecules. Steric effects complement electronic effects, which dictate the shape and reactivity of molecules. Steric repulsive forces between overlapping electron clouds result in structured groupings of molecules stabilized by the way that unlike charges attract and like charges repel.
01Steric hindrance
Steric hindrance is a consequence of steric effects. Steric hindrance is the slowing of chemical reactions due to steric bulk. It is usually manifested in intermolecular reactions, whereas discussion of steric effects often focus on intramolecular interactions. Steric hindrance is often exploited to control selectivity, such as slowing unwanted side-reactions.
Steric hindrance between adjacent groups can also affect torsional bond angles. Steric hindrance is responsible for the observed shape of rotaxanes and the low rates of racemization of 2,2'-disubstituted biphenyl and binaphthyl derivatives.
02Measures of steric properties
Because steric effects have profound impact on properties, the steric properties of substituents have been assessed by numerous methods.
Rate data
Relative rates of chemical reactions provide useful insights into the effects of the steric bulk of substituents. Under standard conditions, methyl bromide solvolyzes 107 faster than does neopentyl bromide. The difference reflects the inhibition of attack on the compound with the sterically bulky (CH3)3C group.
A-values
A-values provide another measure of the bulk of substituents. A-values are derived from equilibrium measurements of monosubstituted cyclohexanes. The extent that a substituent favors the equatorial position gives a measure of its bulk.
| Substituent | A-value |
|---|---|
| H | 0 |
| CH3 | 1.74 |
| CH2CH3 | 1.75 |
| CH(CH3)2 | 2.15 |
| C(CH3)3 | >4 |
Ceiling temperatures
Ceiling temperature (
| Monomer | Ceiling temperature (°C) | Structure |
|---|---|---|
| ethylene | 610 | CH2=CH2 |
| isobutylene | 175 | CH2=CMe2 |
| 1,3-butadiene | 585 | CH2=CHCH=CH2 |
| isoprene | 466 | CH2=C(Me)CH=CH2 |
| styrene | 395 | PhCH=CH2 |
| α-methylstyrene | 66 | PhC(Me)=CH2 |
Cone angles
Ligand cone angles are measures of the size of ligands in coordination chemistry. It is defined as the solid angle formed with the metal at the vertex and the hydrogen atoms at the perimeter of the cone (see figure).
03Significance and applications
Steric effects are critical to chemistry, biochemistry, and pharmacology. In organic chemistry, steric effects are nearly universal and affect the rates and activation energies of most chemical reactions to varying degrees. In some cases, steric effects are necessary to ensure a molecule's stability. In Günther Maier's corset effect, bulky substituents stabilize a molecular core because decomposition would force the substituents closer together. Conversely, substituent attraction can stabilize molecules; these molecules are called "dispersion-stabilized".
In biochemistry, steric effects are often exploited in naturally occurring molecules such as enzymes, where the catalytic site may be buried within a large protein structure. In pharmacology, steric effects determine how and at what rate a drug will interact with its target bio-molecules.
Sources and credits
This article is adapted from the Wikipedia article “Steric effects”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.
Images, from Wikimedia Commons:
- C4R4.png by Smokefoot, Public domain
- Regioselective tritylation.png by Michał Sobkowski, CC BY-SA 4.0
- Methyl group cyclohexane.svg by Calmkelp (talk · contribs), Public domain
- ConeAng.png by Unknown authorUnknown author, Public domain
- Triterbutilamin.JPG by en:user:ProkopHapala, CC BY-SA 3.0
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