The following molecular models are representations of (a)...

The following molecular models are representations of (a) adenine and (b) cytosine, constituents of DNA (deoxyribonucleic acid). Indicate the positions of multiple bonds and lone pairs for both, and draw skeletal structures (gray $=\mathrm{C},$ red $=\mathrm{O},$ blue $=\mathrm{N},$ ivory $=\mathrm{H})$

Key Concepts

Skeletal Structure Representations

Skeletal structures are simplified diagrams used in organic chemistry to depict the connectivity of atoms in a molecule while omitting certain details such as hydrogen atoms bonded to carbon. These representations emphasize the arrangement of the carbon framework, functional groups, multiple bonds, and the placement of heteroatoms, making it easier to visualize and interpret molecular structures.

Lone Pairs on Heteroatoms

Lone pairs are pairs of valence electrons that are not involved in bonding. In heterocyclic compounds, especially those containing electronegative atoms like nitrogen and oxygen, lone pairs play a crucial role in determining the molecule’s shape, reactivity, and ability to form hydrogen bonds or act as electron donors in chemical reactions.

DNA Nucleobases

DNA nucleobases are the fundamental building blocks of DNA that include purines and pyrimidines. They are heterocyclic compounds containing nitrogen and other elements such as carbon, oxygen, and hydrogen. Their specific structures allow for complementary base pairing, which is critical for the double-helical structure and replication of DNA.

Molecular Bonding in Organic Compounds

Molecular bonding in organic compounds involves the formation of single (sigma) bonds and multiple bonds, such as double bonds that include both a sigma and a pi component. Multiple bonds influence the geometry and reactivity of molecules and are important for understanding the electronic structure and chemical behavior of organic molecules.

Transcript

00:01 So we're given the following molecular models or adenine and cytosine.

00:06 And we want to indicate any positions where there are double bonds, an anelone pairs, and also draw the skeletal structures for each.

00:16 So something that would help is if we recall how many bonds each of these atoms normally will have.

00:24 So carbon normally has four bonds.

00:31 Nitrogen normally has three bonds, oxygen normally has two bonds, and hydrogen normally has just one bond.

00:39 So we shouldn't have any double bonds for the hydrogens.

00:42 And also, since we need to talk about lone pairs, normally nitrogen is going to have one lone pair, and oxygen is going to normally have two lone pairs, and the carbon and the hydrogen normally don't have any lone pairs.

00:57 Now, let's go ahead and figure out which of these atoms differ from what we have over there.

01:08 So this nitrogen up top has three bonds, so that works out.

01:14 So that means it would also need a lone pair on it.

01:18 This carbon here only has three bonds, so we'll need to come back to put a double bond between some other atom.

01:27 This nitrogen here only has two bonds, so we'll need to come back and put a double bond.

01:33 This carbon only has three bonds, so it needs something as well.

01:37 This nitrogen only has two bonds, so it'll need an extra.

01:42 This carbon only has three bonds, so it needs an extra.

01:45 This carbon has three bonds.

01:47 This carbon has, or this nitrogen has two, this carbon has three, and then this nitrogen has three.

01:53 So all the ones i put a little green dot on will need a double bond.

02:00 So it seems to be easier to start over here on the left, so let's do that.

02:03 So we'd have a double bond here, then we'd have a double bond between these, a double bond between those, and a double bond there.

02:10 Now if you were to go back and count everything up, you would see everything has the correct number of bonds, and then each of the nitrogen's, we're just going to throw on one lone pair...

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