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Tutor profile: Marcus S.

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Marcus S.
Graduate Student Researcher in Organic Chemistry at Harvard University
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Questions

Subject: Organic Chemistry

TutorMe
Question:

What is chirality, what is a chiral center, and how do I tell if a compound is chiral?

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Marcus S.
Answer:

Chirality is a geometric property belonging to three-dimensional objects. Simply put, an object (including a molecule) is chiral when it has no plane or point of symmetry. The implication of this asymmetry is that a chiral object cannot be superimposed onto its mirror image. Your left and right hands (or ears) are mirror images of each other, and each hand/ear has no plane or point of symmetry. In molecules, this asymmetry manifests physically in that chiral molecules rotate plane-polarized light in one direction. The mirror images of the molecule rotate the light in opposite directions, but virtually all other physical properties stay the same. A chiral or stereogenic center is an atom within a molecule that is locally chiral due to its substituents. A common example is a carbon atom in a molecule that has four different substituents attached to it - you can easily see that there is no way to construct a plane of symmetry to divide such a structure into two equal halves. Molecules are chiral only because they lack a plane or point of symmetry. A common misconception is that molecules with chiral centers are chiral - this is not true! In other words, a molecule with chiral centers need not be chiral, and a molecule without chiral centers can be chiral too, as long as it is asymmetric overall.

Subject: Chemistry

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Question:

What is the difference between a Bronsted-Lowry acid/base and a Lewis acid/base?

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Marcus S.
Answer:

In both cases, acids and bases are behave oppositely from one another, and these two definitions differ in what is being transacted. Bronsted-Lowry acids and bases are defined in terms of proton transfer: a B-L acid tends to release a proton (H+), while a B-L base tends to accept or abstract a proton. The Lewis definition is much more general: a Lewis acid tends to accept electrons (or is electron-deficient); a Lewis base tends to donate electrons (or is electron-rich). The B-L definition is therefore a special case of the Lewis definition. For instance, a B-L acid is prone to donate a proton (H+) because it has an excess of positive charge and wants to get rid of it; from another point of view, such a species would welcome negatively charged electrons to neutralize its positive charge, meeting the Lewis criterion for acids. Try making the analogous argument for bases!

Subject: Basic Chemistry

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Question:

What is a chemical bond?

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Marcus S.
Answer:

Simply put, a bond occurs when two or more atoms share electrons to achieve an electronic arrangement that is mutually more favorable—lower in energy, or more stable—than the individual atoms. This increased stability is why molecules like carbon dioxide (CO2) exist instead of individual carbon and oxygen atoms. Multiple models such as valence bond theory and molecular orbital theory exist as simplified descriptions of chemical bonding. For example, molecular orbital theory describes bond formation as the mathematical linear combination of constituent atomic orbitals. The description of the resulting bonding and antibonding orbitals gives us important insight into how molecules react with each other.

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