A carbon atom possesses four electrons in its outermost orbit. To reach stability, it therefore tends to form four covalent bonds. By sharing these four electrons, it acquires an additional four electrons from other atoms, completing its outermost shell with a total of eight electrons. Atoms that readily share electrons with carbon, thus forming bonds, include hydrogen, oxygen, nitrogen, halogens, and, naturally, other carbon atoms. These bonds are referred to as carbon-carbon bonds, carbon-hydrogen bonds, carbon-oxygen bonds, etc. (Note they are all covalent bonds.)
Chemists define organic compounds as compounds containing carbon-carbon or carbon-hydrogen bonds. Some complex organic compounds typically feature a long carbon chain, where carbon atoms are linked together like beads. Attached to carbons are other atoms or functional groups. In this passage, we will provide a brief introduction to the basic types of organic compounds, beginning with those containing only carbon and hydrogen atoms (hydrocarbons).
Hydrocarbons are also called aliphatic compounds. Hydrogen only has one electron, so carbon-hydrogen bonds are single bonds. And based on their distinct carbon-carbon bonds, hydrocarbons are classified into three types: alkanes, alkenes, and alkynes. Alkanes have single bonds between all carbon atoms, making them saturated. As single bonds are quite strong and do not break easily, alkanesare resistant to chemical reactions. Next comes alkenes, whose carbon atoms form double bonds, making them unsaturated. Alkenes are more reactive than alkanes, and most reactions involving alkenes include the “opening up” of double bonds and the formation of two new single bonds. Alkynes have triple bonds between carbon atoms, making them highly unsaturated, therefore exceptionally reactive.
Let’s now add oxygen atoms into the picture. Oxygen atoms have six outermost
electrons, so they want to form two covalent bonds to achieve stability. Hydrogenoxygen bonds are always single bonds. The bond between a carbon and an oxygen
atom can be either single or double. In the case of a single bond, the oxygen
atom is highly likely to form an additional bond with some other atoms. If it’s a
double bond, the oxygen atom will not be bonded to any other atoms, as it has
no remaining electrons available. When an oxygen atom forms a single bond with
a carbon atom on one side and another single bond with a hydrogen atom on the
other (thus forming the hydroxyl group -OH), the resulting compound is called an alcohol. The hydroxyl group is one of the common functional groups. A functional group is an assembly of specific atoms. Different structural arrangements of these atoms give distinct properties to organic compounds. One of the most well-known alcohols is ethanol. The polarity of the hydroxyl group makes alcohols soluble in water, as the oxygen atom readily forms hydrogen bonds with the hydrogen atoms in water molecules. Alcohols are chemically active and capable of undergoing various reactions, such as dehydration and oxidation.
What about carbon-oxygen double bonds? The group formed by the carbonoxygen double bond is commonly known as the carbonyl group (-C=O). After
carbon atoms form a double bond with oxygen atoms, there are two electrons left
for bonding. One of these electrons needs to form a single bond with the adjacent
carbon atom in the chain. If the last remaining electron forms a single bond with a
hydrogen atom (H), the resulting compound is called an aldehyde (-CHO). The most
common aldehyde is formaldehyde, primarily used in resin production.
If the last electron forms a single bond with a carbon atom (C), the resulting
compound is called a ketone. Ketones typically have higher boiling and melting
points and can participate in various organic reactions. They are extensively used in
industry as solvents, polymers, and in pharmaceutical production.
If the last electron bonds with a hydroxyl group (-OH), it forms a carboxyl
group (-COOH). Compounds containing a carboxyl group are called carboxylic
acids. Carboxylic acids are weak acids that can react with bases to produce salts
and water.
We haven’t covered all the possibilities yet. The structure of carboxylic acids
can also be viewed like this: one oxygen atom (O) forms a single bond with the
carbon in the carbonyl group (-C=O), and at the other end, it forms a single bond
with a hydrogen atom (H). If we replace this hydrogen atom with a carbon atom
(C), so that the oxygen atom is single-bonded to the carbonyl group (-C=O) and
also single-bonded to another carbon, what results is an ester, represented by the
functional group -COO. Esters have high volatility, and generally possess a fragrant
aroma; many fruit peels contain esters.
Next, let’s introduce nitrogen atoms. To have a better understanding, this time
we won’t start from the perspective of carbon atoms but from that of nitrogen
atoms. Nitrogen atoms have 5 electrons in their outermost shell. To achieve stability,
they tend to form 3 covalent bonds. Therefore, ammonia (NH3) (where a nitrogen
atom forms single bonds with 3 hydrogen atoms) is a stable compound. As long as
any one of these H atoms is replaced by an alkyl group (an alkyl group, represented
as R, consists only of carbon and hydrogen atoms, with all single bonds), the
resulting organic compound is called an amine. There are 3 replaceable hydrogen
atoms, so there are 3 types of amine groups: -NH2, -NHR, and -NR2. They are
respectively called primary, secondary, and tertiary amine. Amines are basic and can
react with acids to form salts.
When one of the hydrogen atoms is replaced not by alkyl groups but by carbonyl
groups (-C=O), the resulting compound is called an amide. Similarly, there are three
types of amides: primary, secondary, and tertiary amides. The properties of amides are
significantly different from those of amines; while amines are usually basic, amides are
typically neutral.
Carbon atoms on the carbon chain can also bond with halogen atoms (such as
fluorine, chlorine, bromine, iodine, etc.), forming organic halides.
You might have already thought of it: apart from forming chains, atoms can also
create a unique structure, rings. Common cyclic structures include aromatic, aliphatic,
and heterocyclic rings. The most famous aromatic ring is the benzene ring, consisting
of 6 carbon atoms and 6 hydrogen atoms. Carbons form alternating double and single
bonds, where the electrons shared in the double bonds can move anywhere within the
ring; this is known as conjugated structure. Cyclic compounds containing conjugated
structures are called aromatic rings. Carbon rings other than aromatic rings are called
aliphatic rings(there are both saturated and unsaturated ones). Heterocyclic rings
contain atoms other than carbon, such as nitrogen, oxygen and sulfur.
Many of the mentioned organic compounds can interconvert and transform
into one another through reactions. For instance, alcohols can undergo oxidation
to form aldehydes or ketones, while carboxylic acids and alcohols can
react under acidic conditions to yield esters. For a more detailed visual
representation, you can refer to the infographic diagram on the inside front
cover.
Chemists often use the skeletal formula to represent organic
compounds. In this method of representation, all hydrogen atoms and
carbon-hydrogen bonds are omitted, and every intersections of adjacent
lines represent a carbon atom. Functional groups are usually shown explicitly,
and can either be abbreviated (e.g., -CN) or fully written out (e.g., -C≡N).
Ring structures are depicted using polygons with corresponding number of
lines.
When the stereochemistry of a compound is crucial, various styles
of lines are used to represent its three-dimensional features. Solid lines
represent bonds in the plane of the paper. Solid wedges represent bonds
that point out of the plane of the paper, towards the observer, whereas
dashed lines represent bonds that point into the plane of the paper, away
from the observer.
Certain common functional groups can be represented by specific
symbols. For example, the letter R corresponds to alkyl or hydrocarbon
groups, or any group (if irrelevant to the context and can be omitted); X
represents halogen atoms; M represents metal or alkali metal atoms; Me is
the abbreviation for the methyl group, corresponding to -CH3.
Let’s use an example to illustrate how to read skeletal formulas.
The diagram below is the skeletal formula of the antidepressant drug
Escitalopram (C20H21FN2O). Each turning point or unmarked endpoint
represents a carbon atom. If not specified, each carbon atom is assumed to
be connected to hydrogen atoms (in a way so that it always forms 4 covalent
bonds in total). Can you count all 20 carbon atoms and 21 hydrogen atoms?
The two rings in the lower-left and upper corners each have three carboncarbon single bonds and three carbon-carbon double bonds, making them
benzene rings. The upper benzene ring is attached to the halogen atom
flourine. The lower benzene ring is connected to a -C≡N group(cyano
group). It is aslo linked to a pentagonal heterocyclic ring. All carbon atoms
are omitted, while the distinctive oxygen atom is marked out.
