Lecture notes
ORGANIC CHEMISTRY
- Module
- Chemistry
- Institution
- University College London (UCL)
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ALCANESSaturated hydrocarbonsdue ,to the fact that carbon is bound to the maximum number of
3
hydrogens, i.e. without double bonds.hybridized . They are also called paraffins because they are
not very reactive. Carbon forms 4 covalent bonds while hydrogen only one.
General formula: CnH2n + 2
They belong to a homologous series (family of compounds that react in the same way) in which
each member differs from the next by a methylene CH2 group.
Methane: 1 carbon hydrocarbon. There is
only one possible structure for this alkane.
Ethane: alkane with 2 carbon atoms. Carbon
forms 4 covalent bonds while hydrogen
only one.
Propane: alkane with 3 carbon atoms. There
is only one possible structure for this
alkane.
Butane: alkane with 4 carbon atoms. There
are two possible structures for this alkane.
The two possible forms for butane are butane and isobutane, a branched alkane, isomer of butane.
In this case we speak of isomers, constitutionalbecause they have the same molecular formula but
differ in the way in which the atoms are bonded.
ISO group à H + 2CH3 isobutane is therefore a 4-carbon alkane with an iso group.
ALKYL GROUPS
They are obtained by removing a hydrogen from an alkane. Their
name is obtained by removing the suffix "ano" and replacing it
with "ile", referring to the corresponding alkane.
If hydrogen is replaced by an OH group it becomes an
alcohol, if it is replaced by an NH2 it becomes an amine,
if it is replaced by a halogen it becomes an alkyl halide, if
it is replaced by an OR group it becomes an ether.
● Propyl: it is obtained by removing a hydrogen
from a primary carbon of propane. A primary carbon is a
carbon bonded to only one other carbon.
● Isopropyl: is obtained by removing a
hydrogen at a secondary carbon of propane. A secondary
carbon is a carbon bonded to two carbons.
, ● Butyl and isobutyl: obtained by removing a hydrogen from the primary carbon of butane.
● Sec-butyl: obtained by removing a hydrogen from the secondary carbon of butane.
● Tert-butyl: obtained by removing a hydrogen from the tertiary carbon of butane. Tertiary
carbon is bonded to three other coals.
Primary hydrogens -> linked to primary
carbons Secondary hydrogens -> linked to secondary
carbons Tertiary hydrogens -> linked to tertiary carbons
Cycloalkanes: alkanes with ring conformation, the name is obtained by adding the prefix "cycle"
to the alkane which corresponds to the number of carbon atoms of the ring. They are represented
using segmented structures, and due to the ring shape alkanes have two fewer hydrogens than
their corresponding alkanes.
General formula: CnH2n
Alkyl halides: compounds in which the hydrogen of an alkane is replaced by a halogen. They are
divided into:
● Primary: halogen bonded to primary carbon
● Secondary: halogen bonded to secondary carbon
● Tertiary: halogen bonded to tertiary carbon
The carbon-halogen bond that replaces the carbon-hydrogen bond is formed by the
3 3
superposition of a sp halogen orbital withorbital of carbon. The bond becomes longer and
weaker as the size of the halogen increases.
Usagename: halogen name + suffix “uro” + di + alkyl group.
IUPAC name: are considered substituted alkanes. The halogen name retains the final "o".
Etheri: compounds in which oxygen is bonded to two alkyl groups.
● Symmetric ethers: the two alkyl substituents are the same
● Asymmetric ethers: the two alkyl substituents are different
Usage name: it is formed by the names of the two alkyl substituents, placed in alphabetical order
followed by the word ether.
IUPAC name: An ether is considered to be an alkane substituted by the RO group. The name of the
substituent is obtained by replacing the suffix “ile” of the alkyl groups with “oxy”.
The geometry of oxygen is the same as the geometry of water, an ether can be considered a water
molecule in which both hydrogens are replaced by alkyl groups.
Ex: methyl -> methoxy
Alcoholi: compounds in which hydrogen is replaced by an OH group (functional group), which is
the reactive center of the molecule.
● Primary alcohols: OH group bonded to primary carbon
● Secondary alcohols: OH group bonded to secondary carbon
● Tertiary alcohols: OH group bonded to tertiary carbon
,Usage name: alcohol + alkyl group to which the OH group is bonded
IUPAC name: the functional group is indicated with the suffix "ol", which replaces the "o" of the
corresponding hydrocarbon.
The geometry of oxygen is the same as the geometry of water, in fact an alcohol molecule can be
considered a water molecule in which a hydrogen is replaced by an alkyl group. Oxygen is sp
3 3 3
hybridized . One of the four sp oxygen orbitals overlaps the sp of carbon, one overlaps the s
3
orbital of a hydrogen, and the remaining two sp each contain a lone pair.
Amines: compounds in which one or more ammonia hydrogens are replaced by alkyl groups.
● Primary amines: an alkyl group is bonded to
● nitrogen Secondary amines: two alkyl groups
● are bonded to nitrogen Tertiary amines: three alkyl groups are bonded to nitrogen
Usage name: alkyl group bonded to nitrogen + amine.
IUPAC name: the final “o” of the parent hydrocarbon is replaced by “amine”. The carbon to which
nitrogen is bonded is identified with a number that may appear before “amine” or before the
name of the parent hydrocarbon.
3
Nitrogen has the same geometry as ammoniacal nitrogen.hybridized with one, two or three
hydrogens substituted by alkyl groups.
Quaternary ammonium salts: groups in which nitrogen binds four alkyl groups.
Conformers: different arrangements in the space of the atoms, deriving from the rotation around
the single bond . Through Newmann's projections it is possible to represent on paper the
three-dimensional molecules that derive from the rotation around the 𝜎 bond. Offset conformer
and eclipsed conformer represent the two limit forms. The staggered conformer is of lower
energy, therefore more stable than the eclipsed conformer. Rotation occurs when a certain energy
barrier is overcome.
Hyperconjugation: electronic delocalization deriving from the superposition. Dihedral
angle: angle that is formed, in a Newmann projection, between the bond of the carbon in front and
one of the carbon behind. Identify the angle of rotation of the bond.
Offset conformer D: more stable conformer in which the two methyls are as far apart as possible.
The D conformer is called anti conformer, while the B and F conformers are called gauche
conformers. Between them they have different energies, except the two gauche conformers who
have the same energy. Conformers have different energies due to steric tension, which is the
tension it assumes.
Ring tension -> difference between the planar cycloalkane angle and the ideal bond angle. In
cyclopropane there is less overlap of the orbitals and consequently a weakening of the CC bond,
and this is due precisely to the ring voltage or angular voltage.
CONFORMER OF THE CYCLOESAN
, Conformer to chair: structure almost completely free of tension. All bond angles are 111 ° and
adjacent bonds are offset. In this conformation there are two types of bonds: the equatorial bond
and the axial bond. Each carbon has an axial bond and an equatorial bond.
● The axial bonds are vertical and alternating one
above and one below the ring.
● The equatorial bonds are directed towards the
outside of the ring and are oblique, since the
bond angles are greater than 90 °
Ring inversion: ease of inter-conversion of cyclohexane between two stable chair conformers due
to rotation around the bond CC. When this process occurs, the axial bonds of one form become the
equatorial ones of the other and vice versa.
conformer Boat: It has no angular tension, but is not as stable as the chair
conformer because some CH bonds are eclipsed. The conformer is further
destabilized by the proximity of the hydrogens to the flag pole.
Skewed boat conformers and half chair conformers: to switch from a boat
conformer to a chair conformer, one of the two top coals must move down, to
become the lowest carbon of the chair.
● Conformero with crooked boat is obtained when one of the two coals moves slightly
downwards
● ->.
A further downward movement generates the chair conformer.
If the cyclohexane is monosubstituted the two conformers will not be the same. The substituent
will be in an equatorial position in one conformer and in an axial position in the other. The
conformer with the substituent in the axial position is the most stable. Since the axial bonds on
the same side are parallel to each other, each axial substituent will be close to the others creating
interactions unfavorable interactions 1,3-diaxial
If the cyclohexane is disubstituted we could have different conformations:
● Cis isomer: the two substituents are on the same side
● Trans isomer: the two substituents are on opposite sides
These isomers are called cis-trans or geometric isomers (isomers in which the same atoms are
arranged differently in space) . In this case the cis isomers and the trans isomers each have two
chair conformers with the same stability. On the contrary, the two trans-chair conformers have
different stabilities because one has the two substituents in an equatorial position (more stable)
and one in an axial position.
When two cyclohexane rings are condensed(they have two adjacent carbons in common) each ring
can be considered a pair of substituents of the other ring, which can be either cis or trans. When
dealing with the cis isomer there will be one substituent in the axial position and the other in the
equatorial position. The trans isomer has both substituents in the equatorial position.
STEREOCHEMISTRY
ISOMERS -> compounds with the same molecular formula but with different structures. Isomers
are divided into:
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