Basic principles in organic chemistry: Steric and electronic effects in a covalent bond
The following discussion has been contributed by Saurja DasGupta
Chemical reactions take place as a result of giving, taking and/or sharing of electrons.Â So the different effects which influence the distribution of electrons in a covalent bond of an organic molecule are important for understanding the mechanism of the reactions the molecule undergoes.Â It is important to note that the driving force behind all of these so-called â€˜effectsâ€™ is stability and energy minimization.
A covalentÂ single bond is made up of two paired electrons.Â But in cases when the atoms forming the bond differ in electronegativity Â (electronegative atoms love electrons) it results in a â€˜polarizedâ€™ bond (which means the bonded electrons are shifted towards the more electronegative atom). Â So a C-N bond in CH3NH2 (methylamine) would be polarized as follows:
Î´+Câ€”NÎ´–, where Î´ indicates partial charge, not full shift of charge like an ionic bond. So what basically happens is N pulls the bonded electrons towards it leaving the C slightly positive or electron deficient.Â Now if this C-N bond is a part of a bigger chain like inÂ C3H3-C2H2-C1H2-NH2, then due to this effect, the C next to the N is slightly positive charged. Being positive (electron deficient) it wants more electrons so it pulls the bonded pair of electrons from the C next to it (C2), which in turn becomes slightly positively charged as a result. Now the chain electron distribution looks like:
-C-CÎ´Î´+-CÎ´+-NÎ´–, where Î´Î´+ means less positively charged than Î´+. This relay of charge is called Inductive (I) effect.Â Since this case involves pulling of electrons which start the whole thing, it is termed â€“I effect.Â We can have +I effect as well which is illustrated as follows:
The (CH3)2-CH group also known as the isopropyl group is electron pushing; all alkyl groups can be considered to be electron donating. There is another effect called Hyperconjugation which hasÂ a role to play sometimesÂ in the +I effect of alkyl groups, which we will discuss later.Â So C2 has excess electron density due to the electron pushing of the isopropyl group next to it. C2 having excess electrons push them to C1 making it partially negatively charged too. This effect does not carry beyond 2-3 carbon atoms.
The following list would be helpful for determining the magnitude of inductive effects in different molecules:
- Decreasing order of -I effect of these groups when attached to a molecule:
R3N+ > NO2 > CN > F > Cl > OH > OCH3 > Br > I > -CH=CH2
- Decreasing order of +I effect of these groups when attached to a molecule:
-O (due to Oâ€™s lone pair of electrons) > (CH3)3C- > (CH3)2CH- > CH3CH2– > CH3–
[stextbox id=”info” caption=”Why is this important?â€”An illustration”]
The order of acidities of the molecules are as follows:
CF3-COOH > CH2F-COOH > CH2Cl-COOH > CH3COOH
Again the order of basicities of the molecules are as follows:
(C2H5)2-NH > (CH3)2-NH > CH3-NH2 > NH3
How can these be explained?
Let us discuss the acids first. Â The reaction which makes those molecules acids is
So the more stable the carboxylate ion the easier this reaction will be, because remember all chemical processes try to go in the direction of stability.Â So if we compare the stabilities of the carboxylate ions from all the acids given we can tell which would readily give up the proton according to the reaction shown above.
CF3-COO– has 3 fluorine (F) atoms which are highly electronegative and according to the list given above has a larger â€“I effect than Cl which in turn has a much larger â€“I effect than C as halogens are more electronegative than C. Â Also CF3-COO– has two more electron withdrawingÂ F atoms than CH2F-COO–, so the negative charge is better dissipated in CF3-COO– than CH2F-COO–. So CF3-COO– is more stable than CH2F-COO–. CH2F-COO– again is more stable than CH2Cl-COO–, simply because F has greater â€“I effect than Cl. CH2Cl-COOÂ is more stable than CH3COO– as the latter has no electronegative groups to pull the negative charge away. Â The stabilities of the carboxylates ions (better called â€˜conjugate basesâ€™ of the acids) are in the order :
CF3-COO– > CH2F-COO–> CH2Cl-COO– > CH3COO–.
As the more stable the carboxylate ions easier the deprotonation reaction, hence greater the acidity of the corresponding acid, the order of acidity is indeed :
CF3-COOH > CH2F-COOH > CH2Cl-COOH > CH3COOH
For basicities of the amines, again inductive effect can be used to explain the order. The more C atoms the more is the +I effect (see the list provided). So (C2H5)2-NH has the most electron density on the N atom due to the highest +I effect from the alkyl substituents. Â Across the series the number of substituents decrease and so does the +I effect Â thus gradually decreasing the electron density on the N atom which is Â the basic centre of the donating electrons thus acting as a Lewis base.Â The more the electron density on N, the better Â it can Â donate electrons, being the stronger base in the process. So (C2H5)2-N is the strongest base and NH3 with no alkyl substitutions is the least basic. So the order
(C2H5)2-NH > (CH3)2-NH > CH3-NH2 > NH3
The term though a bit outdated is important for understanding changes in electronic density in a molecule in the presence for other species. This also involves movement of electrons but in this case due to some external agent. For example if a positive charge like H+ is brought near a double bond (say CH2=CH2), the double bond which is electron rich (a double bond has pi electrons, remember?), the bond is polarized towards the proton, which can be shown as follows:
This shifting of electrons or polarization of the covalent bond is termed as Electromeric effect. This case is called Â +E, as the polarization occurs due to the presence of a positive charge. A â€“E effect can be seen when some negatively charged species like OH– attacks a double bond:
Resonance is one of the most fundamental concepts of chemistry with the most applications in organic chemistry. Â Resonance is a phenomenon where a molecule is represented in more than one form when a single Lewis structure cannot represent all of its properties.Â An example is 1CH2=2CH-3CH=4CH2, which from the Lewis structure shown consists of two double bonds and a single bond. So C1-C2and C3-C4 bond lengths should be substantially shorter than that between C2 and C3, but all bonds are found to be of the same length in reality. So the above representation of bonds and electrons is not entirely accurate.
In reality this inadequacy of accurate representation of covalent molecule is inherent in the Lewis model. Â So we use multiple structures to represent the actual molecule which exists in nature and these structures represent one or more propertiesÂ of the actual molecule.Â Philosophically it is like human nature, where we are made p of many different traits, but a person cannot be described by any single one of those traits.
To extend the example above CH2=CH-CH=CH2 can be written as CH2+-CH=CH-CH2– by moving the electrons in the following manner:
These structures are called resonance structures of the main molecule; now we can understand why all the bonds have equal length as structure 2 has a double bond character on C2-C3 1 and 3 have double bonds on C1-C2 and C3-C4. So overall all C-C bonds have some double bond character so the actual representation of the molecule found in nature would be something like:
This structure now would be called a resonance hybrid of all the resonance structures sometimes also referred to as canonical forms.Â An analogy can be used here.
A foreigner comes to India and sees a rhino for the first time…then when he goes back to his country he tries to describe the animal he saw and he does it by saying the animal was a combination of a unicorn (a fairy-tale horse with one horn above its nose) and a dinosaur.Â So we can see that these two animals carry distinctive features of the rhino, so the rhino can be considered to be a resonance hybrid of the two. The most important thing here is that neither the unicorn nor the dinosaur exists in real life but the rhino does; resonance structures do not exist but are merely used to describe the actual molecule-the resonance hybrid which exists in nature. So the movement of electrons shown by the arrows to obtain those resonance structures are also superficial and only drawn for easier understanding.
A few points about Resonance hybrids would summarize the concept:
- Â A resonance hybrid is the actual representation of the molecule.
- It has properties from all the resonance structures.
- It has the least energy of all the resonance structures (that is why it exists in nature) and the structures which have energy close to it contribute the most towards it. This means if X is the hybrid of A, B and C, if C has the lowest energy (or is the most stable) X will look the most like C.
- This extra stabilization of the resonance hybrid is denoted by Resonance energy.
This is very similar to resonance, sometimes referred to a No-bond resonance or Baker-Nathan effect. In case of classical resonance we had seen the involvement of lone pair of electrons and pi bonds (double/triple bonds). In hyperconjugation single bonds are involved in the electron delocalization circuitry. This effect is still not fully understood in detail but would serve the purpose of basic organic chemistry.
The following example would illustrate this effect.
The cation formed is called a carbocation as we will learn later. This positive charge is stabilized by hyperconjugation as follows:
Notice that the sigma bond is involved in resonance and breaks in order to supply electrons for delocalization.
These effects are very significant in organic chemistry and biology.Â Most books would deal with this effect in a very sketchy way, but it is important to understand the basis of this effect. The word steric is derived from â€˜stereosâ€™ meaning space. So this effect is manifested when two or more groups or atoms come in close proximity to each other (precisely within each otherâ€™s van der Waals radii (definition of van der Waals radii can be found in any standard textbook)) and result in a mutual repulsion.Â This makes the molecule unstable. The situation can be compared to a crowded bus or train where each passenger stands touching the other one and there is collision, one steps on the otherâ€™s feet, hits one another with elbows and so on and so forth. Itâ€™s clearly not a very pleasant scenario! It is the same things with molecules. So sheer bulk of the atoms or groups and their proximity can have serious implications. The usual physical clash between groups, almost always is accompanied by an electronic component as well. This is called stereoelectronic effect, which is not the same as the electronic effects discussed above and does not carry have an effect on some other part of the molecule like inductive and resonance effects. When the two atoms get to close, into each otherâ€™s van der Waalâ€™s radii, the electron cloud surrounding each atom repel each other leading to a lot of destabilization. Steric effect affects different properties of molecules, like acidity, basicity and general reactivity. In biological systems where everything occurs in the level of angstroms and in a very precise manner, steric effects even due to tiny H atoms can result in the improper folding of proteins, leading to serious diseases like Alzheimerâ€™s Disease.1 Steric clashes can leadÂ to improper DNA replication resulting is destruction of genetic information and hence to a plethora of genetic diseases including cancer.2
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A substitution reaction on a halide by a hydroxide does not work in this case because of steric hindrance. The second figure represents the same reaction, with spheres replacing the alkyl groups to show the spatial perspective.
So sterics can help us rule out certain reaction mechanism and help us predict the reactivity of certain molecules in certain reactions.[/stextbox]