Organic Chemistry Notes

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A major part of the DAT organic chemistry section is knowing your reactions. I have constructed the following reaction cheat sheet to expose you to every reaction that could possibly show up on the DAT.

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At the beginning of the 18th century, all naturally occurring substances were divided into two main classes: organic and inorganic based on their source. The compounds obtained from living organisms are called organic compounds. eg. sugar obtained from sugarcane. The compounds obtained from non-living sources like minerals and rocks were called inorganic compounds.

By the middle of the 19th century, there was a common belief that organic compounds were produced only in living organisms under the influence of a natural force called vital force. This hypothesis is known as the vital force which was given by Swedish chemist, Berzelius. This theory means organic compounds could not be prepared artificially.

This result indicates that organic compounds could be prepared in the lab. Urea is the first organic compound prepared in the lab and it is the first step in the rejection of vital force theory. After this break, many organic compounds are prepared. Kolbe synthesized acetic acid, Hennel synthesized ethyl alcohol, and Berthelot synthesized methane. Nowadays more than 95% of organic compounds are obtained by synthesis.

By detailed investigations, organic compounds indicate that all organic compounds contain covalently bonded carbon which almost always contains hydrogen. Organic compounds also contain elements like oxygen, nitrogen, sulphur, halogens, phosphorous, and some metals.

Hence, organic compounds are hydrocarbons ( compounds having carbon and hydrogen) and their derivatives. The study of organic compounds is called organic chemistry.

Generally, an organic compound contains two part-one hydrocarbon part and another functional group part. The functional group is active and the hydrocarbon part is inactive. Organic compounds are mostly covalent. Hence only a certain portion actively participates in the chemical reaction.

The functional group is defined as the atom or group of atoms that determine the characteristic properties of an organic compound.

A homologous series is a series of similarly arranged compounds containing the same functional group in which the adjacent member differs by a methylene (-CH2-) unit. Each member of the homologous series is called homologue and this process is called homology.

i. Empirical formula: The simplest formula that represents the simplest whole-number ratio of the number of atoms of all elements present in one molecule of the compound. eg. The empirical formula of glucose is CH2O whereas its actual formula is C6H12O6.

The number which is used for rating the quality of petrol is called the octane number. It is the percentage of the mixture by volume of n-heptane and iso-octane.

n-Heptane has a low octane number and Isooctane has a high octane number.

Carbon is able to combine with many different molecules including hydrogen which make up the basis of organic life. They can combine to make polymer chains, made up of c-c bonds, through a process called catenation

A homologous series is a series of compounds all of the same family which each differ by on structural unit. This allows us to group together the main compounds which exist in chemistry, into groups which have similar physical properties.

For each successive structural unit, there will be a progressive increase in surface area and molecular mass. These are the factors which affect boiling point, therefore increasing these leads to an increase in boiling point

Plotting number of carbon atoms vs boiling points gives a curved trendline, which is known in maths as the harmonic series. The reason it takes this shape is because each increase in structural unit there is a proportionally smaller effect on boiling point.

This is a compound with the molecular formula \(C_6 H_6 \) Benzene is cyclic and forms a hexagon structure, where each carbon has one hydrogen. Each carbon has one double bond to another carbon and also one single bond.This creates a resonance structure

This is a process where a halogen becomes a free radical and then substitutes for a hydrogen in the alkane. This process is a chain reaction and has three stages: initiation, propagation and termination.

Alkenes are more reactive than alkanes because they have unsaturated double bonds. This means that the double bond can become a single bond leaving space for the addition of other molecules. This is called an addition reaction.

Addition polymerisation is when the alkene combines with other alkenes through an addition reaction to form long chains called polymers. Polymers are very important because they are used to create plastics such as PVC of PTFE.

Like alkanes alcohols can undergo combustion reactions. You need to know how to write the equations for complete combustion, which is easy if you know that the products are again \(CO_2\) and \(H_2 O\)

Alcohols can be oxidized into other functional groups, depending on whether they are primary, secondary or tertiary. An oxidizing agent such as potassium dichromate (K2Cr2O7) is used as oxygen does not split easily.

To obtain an aldehyde from oxidizing a primary alcohol instead of a carboxylic acid the aldehydes must be removed from the reaction as soon as they are formed to prevent them oxidizing further. This can be done by taking advantage of the aldehydes lower boiling point compared to the alcohol. Here the alcohol is added to a boiling oxidizing agent so that as soon as the aldehyde forms it boils and can't be oxidized any further. Then a liebig condenser (Long tube with cold water) is add so that when the aldehyde gas passes through it, it will condense and can be collected.

The carboxylic acid can be obtained naturally, but to speed it up a boiling oxidizing agent is used and then a reflux condenser is added so that the evaporated aldehyde condense and goes back to the oxidizing agent, until it is fully oxidized.

Esterification is when an alcohol and a carboxylic acid combine to form an ester. This reaction is a subcategory of condensation reactions, which are reaction that involve two molecules joining to form a large molecule. These reactions have steam as a by-product which is why they are called condensation reactions.

In Halogenoalkanes the electronegative halogen forces the connecting carbon to have a positive charge. This makes it possible for Nucleophiles to replace the halogen, in a process called nucleophilic substitution.

In benzene, the ring structure gives it areas of high electron density above and below (why this happens is HL knowledge). Electrophiles are attracted to these negatively charged areas and can replace the hydrogens in benzene. This is electrophilic substitution

General Organic Chemistry (GOC) covers the fundamental concepts in the vast field of organic chemistry. A strong understanding of the concepts that come under GOC plays a vital role in the study of relatively advanced topics (such as the mechanisms of named reactions). Keeping this in mind, all general organic chemistry concepts have been explained in a concise, informative manner in this article. Click the link below to download the notes provided in this article in PDF.

Adequate reaction conditions facilitate the formation of an intermediate from the chemical reaction between the reactants. These intermediates are generally unstable and quickly react further to yield a product.

The introduction of an electron-rich or electron-deficient species to a carbon chain results in the formation of a permanent dipole. This effect is called the inductive effect since the dipole is induced by the difference in the electronegativities of the atoms in the molecule.

The direction of the dipole moment depends on the electronegativity of the attacking species (relative to the carbon atom it bonds with). Note that the partial charge (positive or negative) gained by the carbon atom is relayed through the carbon chain, as illustrated below.

-I effect: The -I effect, also known as the negative inductive effect, occurs when an electron-withdrawing group is introduced to a chain of atoms. This results in the transmission of a positive charge through the chain. Example of -I species: Halogens, such as fluorine and chlorine.

Answer: The compound with the methyl functional group (bottom) is the most stable since the CH3 group is a +I species, which increases the stability of the carbocation. Since fluorine and chlorine are electron-withdrawing -I groups, they decrease the stability of the carbocation by increasing the magnitude of the positive charge on it. However, the compound containing fluorine is less stable than the one containing chlorine (because fluorine is more electronegative and, therefore, more electron-withdrawing).

The electromeric effect involves the complete transfer of pi electrons to one of the bonded atoms in an organic compound. It is a temporary effect that occurs due to the presence of an attacking reagent (an electrophile or nucleophile that triggers the formation of a dipole in the organic compound). The electromeric effect subsides when the attacking reagent is removed from the system.

+E Effect: When the attacking reagent is an electrophile, the pi electrons shift towards the attacking reagent, which binds to the atom that retains the bond pair (and is negatively charged).

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