Organic Chemistry Revision:
Organic compounds reflect the exceptional ability of carbon to form covalent bonds with other carbon
atoms to form; chains, branch chains and rings.
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Straight chain compounds are called aliphatic compounds
Ring compounds are called cyclic compounds
Also reflect high but stable valence of 4
Aromaticity: In organic chemistry, the structures of some rings of atoms are unexpectedly stable.
Aromacity is the chemical property in which a conjugated (alternate single and double bonds) ring of
unsaturated bonds, lone pairs or empty orbitals exhibit a stabilization stronger than would be expected
by the stabilization of the conjugation alone. Also considered a manifestation of cyclic delocalization and
of resonance.
Benzene:
Therefore Benzene is Aromatic i.e stronger than expected. This is due to its alternate single and double
bonds i.e its (cyclohexatriene) state. With its delocalized electrons this allows inversion, (switching
positions) of these bonds. Benzene is thus a more stable molecule.
Aliphatic: aliphatic refers to non-aromatic organic compounds. The carbon atoms are linked in straight
chains, branched chains or rings (such as cyclohexane) and includes both saturated and unsaturated
compounds.
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Note: The term ‘alicyclic’ refers to an aliphatic compound which is non-aromatic but in a ring
formation.
Elements in organic chemistry:
Carbon (C) always has a valency of four.
Other widely used elements found in organic compounds are (this list is not exclusive):
- Hydrogen (H)
- Sulfur (S)
- Oxygen (O)
- Fluorine (F)
- Nitrogen (N)
- Chlorine (Cl)
- Bromine (Br)
- Iodine (I)
Formula in Organic Chemistry:
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Astatine (At)
1. Empirical: gives the simplest whole number ration of atoms present
2. Molecular: gives the total number of atoms in one molecule
3. Structural: Shows how the atoms are linked together in a molecule (does not give molecular
shape)
Natural division in Organic Chemistry: Organic chemistry is simplified by natural division of closely
related compounds.
Each family is called a homologous series. All compounds in the same homologous series
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Can be prepared by similar methods
Have similar chemical properties
Exhibit a gradual change in physical properties (progressively along the series)
Can be represented by a general formula (empirical formula)
Have regularly increasing
relative
molecular
masses.
Functional groups: Functional
groups are specific groups of
atoms within molecules that are
responsible for the characteristic
chemical reactions of those
molecules/characteristic
properties of the compounds in a
homologous series.
Some functional
shown to the left
groups
are
Alkanes: Empirical formula = CNH2N+2
*Number of carbon atoms increasing from Methane (1) to Heptane (7)
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Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Notes on the properties of Alkanes
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Longer the carbon chain, higher the melting/boiling point.
With more branches boiling point is lower due to weaker vanderwall force
C
C
C
C
-
Above shows the structure of the hydrocarbon backbone
Manipulation of the above model shows that it is most stable due to strong covalent bonds
directed towards the corners of a tetrahedron i.e the (sp3 hybridization or VSEPR model)
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Above shows the physical properties of some Alkanes
Alkanes are colourless
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methane
to
butane
are
colourless
gases
(propane and butane are easily condensed under pressure & are commonly sold as liquids)
Alkanes containing 5 carbons up to about 19 are colourless liquids
(petrol & kerosene are mixtures of liquid Alkanes, dye is added to the fluids for safety reasons)
Alkanes with more than about 20 carbon atoms are colourless, waxy solids
(paraffin wax is a mixture of solid Alkanes)
Alkanes are less dense than water
-
Density increases with increasing molecular mass
Simple Alkanes have low melting and boiling points.
Alkanes are non-polar so only weak intermolecular forces act between the Alkanes molecules
(Van der Waal's Forces/London Forces/Dispersion Forces/Weak Intermolecular Forces)
Melting and Boiling Points increase as the molecular mass increases
Alkanes are insoluble in polar solvents like water
Alkanes are relatively unreactive
(They will combust: commonly used as fuels since large amounts of energy are released, the
longer the chain, the more bonds are broken, the greater the energy released)
(will undergo halogenation by substitution reaction in the presence of ultra-violet light)
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Above Diagram represents the ‘cracking’ of medical paraffin
Alkenes: Empirical formula = CnH2n
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alkenes are hydrocarbons: made up of only carbon and hydrogen atoms
there is a double bond between 2 carbon atoms
general formula: CnH2n
name ends in "ene"
prefix determined by the number of carbon atoms in the chain:
ethane C2H4
propene C3H6
butane C4H8
pentene C5H10
hexane C6H12
heptene C7H14
octane C8H16
nonene C9H18
decene C10H20
Physical state
- The first lower member like ethene, propene and butene are colorless gases. Alkenes with five
to fifteen carbon atoms are liquids and higher ones are solids at ordinary temperatures. Alkenes
have characteristic smell.
Density
- Alkenes are lighter than water.
Solubility
- Alkenes are insoluble in water and soluble in organic solvents such as benzene, ether etc.
Boiling point
- The boiling points of alkenes gradually increase with an increase in the molecular mass (or chain
length). This indicates that inter-molecular attractions become stronger with an increase in the
size of the molecule. Branched chain alkenes have lower boiling points than the corresponding
straight chain isomers. This is because cis-isomers are polar molecules and have higher boiling
points when compared to the trans-isomers. .
- cis-2-butene (b.p.= 3.7°C) and trans-2-butene (b.p.= 1°C)
Melting point
- The melting points of alkenes increase with an increase in the molecular mass. The transisomers fit well into the crystal lattice due to their symmetrical structure. This results in their
melting at higher temperatures as compared to the cis form.
General chemical properties of alkenes
- Alkenes are more reactive than alkanes due to the presence of a double bond. The carboncarbon double bond consists of a strong bond and a weak p bond. The typical reactions of
alkenes involve the breaking of this weaker p bond, viz., and formation of two sigma (s) bonds.
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Such reactions are called addition reactions and are initiated by an electrophile, proceeding
through ionic mechanism. However, some addition reactions proceed through free-radical
mechanism.
Higher alkenes contain a long chain of carbon. That part of the chain that forms an alkane-like
structure (consisting of C-C bonds), may undergo substitution reaction as also shown by alkanes.
Some characteristic reactions shown by alkenes are described below:
Combustion
- Alkenes, like Alkanes, are highly combustible. Alkenes burn with a luminous flame to give carbon
dioxide and water. The flame becomes luminous because of the higher carbon content of
alkenes than Alkanes. Their combustion reactions are exothermic.
-
Due to the luminosity of the flame, the lower alkenes may be used as illuminants.
Alkynes: Empirical formula = CnH2n-2
Ethyne (Acetylene)
Propyne
1-Butyne
1-Pentyne
1-Hexyne
1-Heptyne
1-Octyne
1-Nonyne
1-Decyne
2-Butyne
2-Pentyne
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HCCH
HCCCH3
HCCCH2CH3
HCC(CH2)2CH3
HCC(CH2)3CH3
HCC(CH2)4CH3
HCC(CH2)5CH3
HCC(CH2)6CH3
HCC(CH2)7CH3
CH3CCCH3
CH3CCCH2CH3
The alkynes are the third homologous series of organic compounds of hydrogen and carbon,
where there is at least one triple-bond between the atoms in the molecules.
Physical properties
-
Alkynes are compounds which have low polarity, and have physical properties that are
essentially the same as those of the Alkanes and alkenes.
1. They are insoluble in water.
2. They are quite soluble in the usual organic solvents of low polarity (e.g. ligroin, ether, benzene,
carbon tetrachloride, etc.).
3. They are less dense than water.
4. Their boiling points show the usual increase with increasing carbon number.
5. They are very nearly the same as the boiling points of Alkanes or alkenes with the same carbon
skeletons.
Isomers
Two Types:
1. Structural Isomers- moving the functional group or a part of the molecule around to make a new
configuration. Ensure that all possible movements have been done and that none are
equivalent. Odd numbered ‘n’ has less isomers than even numbered ‘n’
2. Stereoisomers or Geometrical isomers- The only type we need to know is cis and trans. ONLY
occurs in alkenes along the double bond. Cis “on the same side” and trans
“across” or “on the other side” referring to the functional group. Appended to the name,
normally at the front.
Alcohols
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Cn H 2n 1OH
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BP and MP are both exponentional- Why? Always Liquid at RTP due to OH group, stronger
intermolecular bonding, O is highly electronegative, dipole-dipole forms WHEREAS alkanes are
non-polar, low electronegativity, Van der Waals only
Secondary and tertiary alcohols have higher shielding- Harder to oxidize, more soluble- OH
groups is hydrophilic, more polar
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1O Alcohol  Aldehyde  Carboxylic  Acid
2O Alcohol  Ketone
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Oxidisation of alcohols is used to produce many other hydrocarbons
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Metal e.gCopper Oxide  Alcohol  Metal  Aldehyde  Water
CuO(s)  CH 3OH (aq)  Cu(s)  CH 2O(aq)  H 2O(1)
Acidified KMnO4
Sodium
HCl
Solubility
Shielding
Reactivity
PRIMARY
Brown precipitate, Clear
liquid
Fast Reaction, produces
hydrogen gas
Bubbles, 2 layers
X
X
High
SECONDARY
TERTIARY
Brown precipitate, 2 X
layers
Slow bubbles
X
Slow Bubbles
X
Slight
Some
X
High
High/Polar
X
Esterification
CarboxylicAcid  Alcohol É Ester  Water
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Sulphuric acid catalyst
Equilibrium reaction
Produces an odour
BP graph plateaus due to phase changes
Improving yield- less impurities, higher [], Isomerism or increased temperature or catalyst
Distillate contains- Acid, Water, Ester, Alcohol
THEREFORE need fractional distillation
Refluxing= vaporizing, molecules are broken up then recondensing to form products
Balance towards products
MUST BE GENTLE or overbalance and switch to form more reactants
BP Order: Sulphuric acid (therefore, will still be present), Carboxylic acid, Ester, Water, Alcohol
(Highest to lowest)