Comprehensive Review for the MCAT Organic Chemistry Organized by Officially Tested Topics. Roberts, John D. Benjamin, Inc., Menlo Park, CA. Steric and electronic effects in a covalent bond – Open Teaching Project. The following discussion has been contributed by Saurja Das. ![]() Principles of Colour Chemistry 1.1 Basis for colour Unlike most organic compounds, dyes possess colour. This is the first semester in a two-semester introductory course focused on current theories of structure and mechanism in organic chemistry, their historical. Cooling Water Management Basic Principles and Technology By: Timothy Keister, CWT Fellow, American Institute of Chemists Certified Water Technologist, Certificate #90. Bases react with acids to neutralize each other at a fast rate both in water and in alcohol. When dissolved in water, the strong base sodium hydroxide ionizes into. Analytical Chemistry. Structure Determination of Organic Compounds - Tables of Spectral Data E. Bühlmann, M. Badertscher. ![]() ![]() Gupta. Chemical reactions take place as a result of giving, taking and/or sharing of electrons. Inductive effects. So what basically happens is N pulls the bonded electrons towards it leaving the C slightly positive or electron deficient. 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- C. This relay of charge is called Inductive (I) effect. There is another effect called Hyperconjugation which has a role to play sometimes in the +I effect of alkyl groups, which we will discuss later. 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: R3. N+ > 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 O’s lone pair of electrons) > (CH3)3. C- > (CH3)2. CH- > CH3. CH2– > CH3–. So CF3- COO– is more stable than CH2. F- COO–. CH2. F- COO– again is more stable than CH2. Cl- COO–, simply because F has greater –I effect than Cl. CH2. Cl- COO is more stable than CH3. COO– as the latter has no electronegative groups to pull the negative charge away. The more C atoms the more is the +I effect (see the list provided). So (C2. H5)2- NH has the most electron density on the N atom due to the highest +I effect from the alkyl substituents. So (C2. H5)2- N is the strongest base and NH3 with no alkyl substitutions is the least basic. So the order(C2. H5)2- NH > (CH3)2- NH > CH3- NH2 > NH3 is justified. 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: Figure 2. This case is called +E, as the polarization occurs due to the presence of a positive charge. A –E effect can be seen when some negatively charged species like OH– attacks a double bond: Figure 3. 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. Resonance structures. 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: Figure 5. Resonance hybrid. This structure now would be called a resonance hybrid of all the resonance structures sometimes also referred to as canonical forms. 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: 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. Hyperconjugation. 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. Figure 6. Formation of a carbocation. The cation formed is called a carbocation as we will learn later. This positive charge is stabilized by hyperconjugation as follows: Figure 7. Hyperconjugation. Notice that the sigma bond is involved in resonance and breaks in order to supply electrons for delocalization. Stericeffects. These effects are very significant in organic chemistry and biology. The word steric is derived from . So this effect is manifested when two or more groups or atoms come in close proximity to each other (precisely within each other’s van der Waals radii (definition of van der Waals radii can be found in any standard textbook)) and result in a mutual repulsion. 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 other’s feet, hits one another with elbows and so on and so forth. It’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 other’s van der Waal’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 Alzheimer’s Disease. Steric clashes can lead to improper DNA replication resulting is destruction of genetic information and hence to a plethora of genetic diseases including cancer. The second figure represents the same reaction, with spheres replacing the alkyl groups to show the spatial perspective. Figure 8. An illustration of steric effects. So sterics can help us rule out certain reaction mechanism and help us predict the reactivity of certain molecules in certain reactions.
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