The scent of rain is one of the things that comes to our mind when we think about a rainy day. The smell is so intense in air that you can not miss it. Geosmin is responsible for this strong scent that is present in air when rain falls after a hot or dry weather.
Geosmin is produced by several class of microbes especially the Streptomyces. Geosmin is released when these microbes die. Geosmin translates to “Smell of Earth”. It is an organic substance that also is responsible for the earthy taste of beetroot. Geosmin is a colourless liquid which boils at 270oC. Humans are extremely sensitive to the smell of Geosmin and can detect it even at extremely low concentration.
Geosmin can be synthesized in the lab from a sequence of reactions starting with a condensation reaction between 2-methyl-cyclohexanone and ethyl vinyl ketone.
Isn’t it fascinating to know how chemistry is a part of everything around us?
Use in Chemistry: Conversion of Silver salts of carboxylic acids to alkyl halides
The widely believed mechanism involves the Silver salt of the carboxylic acid reacting with bromine to form acyl hypobromite which undergoes homolytic bond cleavage to produce bromine radical and a carboxylate radical. This carboxylate radical then releases a carbon dioxide molecule to produce an alkyl radical. This alkyl radical combines with the bromine radical from the acyl hypobromite to form alkyl bromide.
A compound A crystallizes in a hexagonal close packing system. If the compound is made of X and Y in which X occupies all the lattice position of hcp layers and Y occupies two third of the octahedral voids, what will the formula of A be?
Hydroxycitric acid (HCA) is a derivative of citric acid and is found in variety of tropical plants. It is found in Garcinia Indica commonly known as kokum and is used in Indian & Thai food as a flavoring agent.
Earlier experiments with HCA on mice showed that when mice were administered with HCA along with their food, it made them eat less.
If our food contains large amounts of carbohydrates, the ones that are in excess get converted finally into fat through enzymes. Any chemical that slows down this enzyme will help in accumulation of less fat. HCA, like citrate (natural chemical in our body) slows down the enzymes that convery carbohydrates into fatty acids which in turn become fat. HCA is also found to make liver release large amounts of sugar in the blood and this high sugar level in blood may suppress hunger.
Though the action of HCA was observed in mice, the same effect is yet to be proven in humans. While some claim HCA to be effective in reducing food intake leading to weight loss some claim that HCA causes serious health problems inculding liver damage and cardio vascular disorders.
A negatively charged species can act both as a base and also as a nucleophile. When an alkyl halide reacts with a negatively charged species there are two possible outcomes; elimination and substitution. If the outcome is elimination the negatively charged species has played the role of a base and if the outcome is substitution then the negatively charged species has acted as a nucleophile.
If we describe a chemical reaction in terms of bonds it is nothing but the breaking of old bonds and formation of new bonds. In a reaction, if a species forms a bond with (or abstracts) a proton it is acting as a base and if it forms a bond with (or attacks) a carbon atom it is acting as a nucleophile.
What is the fundamental difference between a base and a nucleophile?
Two major parameters control a chemical reaction. They are Thermodynamics and Kinetics. While thermodynamics helps us predict the spontaneity of a reaction at the given conditions, kinetics helps us understand the mechanism of a reaction.
Let us consider the following equilibrium;
As all the acid base reactions reach equilibrium after sometime, the extent to which the equilibrium has moved in the forward or backward direction will tell us about the stability of the reactants and products.
There are two bases in this equilibrium; AcO- & HO-. To compare the strengths of these two bases we can look at the their stability. A strong base is the one that is more unstable and easily gains stability by abstracting a proton.
If AcO- is more unstable than HO- the equilibrium will be more shifted in forward direction and if HO- is more unstable than AcO- then the equilibrium will be more shifted in backward direction. The direction in which the equilibrium is shifted can be known from the equilibrium constant.
For the example taken, the equilibrium constant is equal to Kw/Ka which is in the order of 10-14/10-5 = 10-9 (Ka of HOAc = 1.8 * 10-5). The small value of equilibrium constant shows that the equilibrium is highly shifted in the backward direction. So, HO- should be more unstable than AcO- and hence be a stronger base.
The strength of a base will be determined from the equilibrium constant which is in turn related to the free energy change of a process.
The above discussion shows that basic strength is a Thermodynamic Factor.
Let us consider the following two reactions;
Both the reactions are performed under identical conditions. Reaction (i) is over in 7 minutes and reaction (ii) is over in 3 minutes. With this information if you have to identify the stronger nucleophile among HO- and CN- and you are most likely to identify CN- to be the stronger nucleophile almost as a reflex action.
Yes, CN- is the stronger nucleophile among the two because the reaction involving it as a nucleophile gets over faster or in other words CN- attacks the electrophilic carbon faster than HO- does.
Nucleophilic strength is not usually related to the charge density on the negatively charged species (charge density is important to predict basic strength) but on how fast can it attack the electrophilic centre.
Nucleophilic Strength is a Kinetic Factor.
Any parameter that makes the nucleophile slow in attacking the electrophilc centre will be decreasing its strength as a nucleophile. For example, the nucleophilc strength of an anion like I- is decreased by polar protic solvents.
Polar protic solvents release protons and these protons solvate the anions and make it difficult for them to move. This reduced the speed with which they attack the electrophilic centre and hence behave as weak nucleophiles.
Aspartame is an artificial sweetener. It is used as a substitute for sugar in food and some beverages. It is the methyl ester of the dipeptide of L-aspartic acid and L-phenyl alanine. Both L-aspartic acid and L-phenylalanine are natural amino acids. It is said that aspartame is almost 200 times sweeter than sugar. This property allows us to have the same amount of sweetness as sugar but will a little quantity. As small quantity of aspartame is used, its calorific contribution becomes insignificant.
Aspartame gets hydrolysed into methanol and its constituent amino acids when hydrolysed in highly acidic conditions and also at elevated temperatures. There has been several articles on research work that shows some serious side effects of aspartame due to this break down especially due to the production of methanol. Methanol can cause permanent blindness even when consumed in little quantity.
Mr. Stanley was missing some money that he had kept in this cup board. This is the third time that he was missing money. In earlier occasions he blamed himself for having lost the money. But, this time he knew for sure that something is wrong. Someone is stealing money from him. “Who could it be?”, he asked himself. He made a list of suspects. It read 4 names. Alex, Charles, Robin and Peter.
Mr. Stanley wanted to catch the culprit during the act. He thought of a wonderful plan. He fetched the washing soda from his laundry room. He then took a little of the washing soda and spread it over some of the notes he had. He ensured that the amount was just enough to catch the culprit. He then left the notes in the same place as earlier and left the room.
Mr. Stanley is using a procedure all chemists do during a titration. During a titration, an indicator is used to indicate the end of the reaction that is being performed. The indicator can be added additionally to the reaction mixture to show completion of the reaction. In some reactions one of the products can act as an indicator. Substances commonly used as an indicator exhibit different color in different medium. Phenophthalein, a common indicator, becomes pink in a basic solution and is colorless in acidic solution.
Like other acid base indicators, phenolphthalein has different colors for its acidic (unionized) and basic (ionized) forms. It is colorless in acidic form and pink colored in basic form
When Mr. Stanley returned to his office after 10 minutes, he observed that the notes have gone as he had expected. He immediately summoned his four suspects to his office. He told them about the frequent disappearance of money from his office. All the four were surprised to hear that there has been a series of theft at their workplace. Mr. Stanley said, “I laid a trap this time to catch the culprit.” There was no nervousness or change of expression on anyone’s face. They still seemed surprised.
"Show me your Palms", he said. All the four extended their hands to show their palms. He then sprayed a solution on phenolphthalein on everyone’s palms. All but Robin’s palms were colorless. The washing soda (Sodium Carbonate) had made phenolphthalein change its colour to pink.
Robin, suspended from work, after being caught Red (pink to be precise) Handed is doing social service to compensate for all the money he had stolen.
Note: Please DO NOT handle chemicals with bare hands. Always be protected from chemicals.
Amino Acids have both amino group as well as a carboxylic acid group. The pKa values of both the groups are well separated that at any given pH a molecule of amino acids carries some charge. For example at a pH as low as 1, the amino group will be protonated and the net charge of the molecule will be positive and at a pH as high as 12 the acidic hydrogen atoms will be removed and the net charge on the molecule will be negative. So, at normal biological pH, most amino acids have the amino group protonated and the carboxylic acid group deprotonated. This ionic structure is called the zwitter ion.
The various forms that an amino acid takes with increase in pH is shown below.
At a lower pH (pH = 2) the amino acid is positively charged and as we increase the pH and move to a higher pH (pH = 12) the amino acid becomes negatively charged. At a lower pH the concentration of the cation (A)is more than that of the anion (C) and as the pH increases the concentration of the anion also increases and becomes more than that of the cation. This implies that at a particular pH the concentration of both the cation and the anion should be equal. This pH at which the concentrations of both the cation and anion are equal is called as the Isoelectric Point of the amino acid.
Generally isoelectric point of an amino acid is the average of both the pKa values.
Ka1 = ([B][H+])/[A]
So, [A] = ([B][H+])/Ka1
Ka2 = ([C][H+])/[B]
So, [C] = (Ka2 [B])/[H+]
At Isoelectric Point, [A] = [C]
([B][H+])/Ka1 = (Ka2 [B])/[H+]
Rearranging the above expressions we get the following
[H+]2 = Ka1 * Ka2
or pH = (pKa1 + pKa2) / 2
There are some amino acids which have an acid or basic side chain which will lead to an additional Ka value. In such cases, the average of the pKa values on either side of the zwitter ion is the isoelectric point.
In the above case Isoelectric point is the average of pKa2 and pKa3 values.
In the above case Isoelectric point is the average of pKa1 and pKa2 values.
Among MgSO4 and BaSO4 which one is more soluble in water at room temperature? Why?
MgSO4 is more soluble than BaSO4 in water at room temperature because among the sulphates of alkaline earth metals the decrease in hydration energy down the group is more rapid than the decrease in lattice energy down the group.
Umpolung is a phenomenon in which the polarity of an atom (usually the carbon) is reversed. This opens up reactions on a functional group which is otherwise not possible. Umpolung is a German word meaning reversed polarity.
Two common reactions that showcase umpolung are benzoin condensation and reactions involving carbonyls and dithianes.
Benzoin Condensation is a reaction between two aromatic aldehydes catalysed by alcoholic KCN. The reaction begins with the nucleophilic attack of the cyanide ion on the carbonyl group which is followed by a rearrangement of the intermediate formed leading to reversal of polarity at the carbonyl carbon. The carbonyl carbon which is otherwise positive becomes negative after the rearrangement.
The carbon atom which is now negatively charged acts as a nucleophile and atatcks another molecule of the aromatic aldehyde leading to the formation of the product after a couple of steps including the one that eliminates the cyanide ion.
Ethylene diol is generally used to protect carbonyl compounds from undergoing any unwanted reaction in certain reactions. If the sulphur anolog (ethane-1,2-dithiol) of ethylene diol is used in protecting the carbonyl group, it opens up an opportunity of reversing the polarity of the carbonyl carbon which is otherwise not possible with ethylene diol.
The carbonyl carbon can carrying a negative charge is favoured as the negative charge can gain stbility by resonating in the vacant orbitals of sulphur atom which is not possible with oxygen atom in its place.
Limonene is an optically active hydrocarbon whose D-isomer possesses a strong smell of oranges. The name suggests it to be an alkene and since it is found in large concentration on the peel of lemons, it is named limonene. It is a cyclic terpene and is used in the synthesis of carvone which is used in the food industry and also in the flavour industry.
The fact that limonene cracks to form isoprene at very high temperatures suggsest that it is a product of the Diels Alder reaction between two isoprene units.
Limonene is also used in food industry and also in some medicines.
The aldehyde molecule is attacked by the hydroxide ion which produces an unstable dianion intermediate in the basic medium. This intermediate then loses a hydride ion which is transferred to another molecule of the aldehyde which in turn is converted into an alkoxide ion. The reaction is a third order reaction; second order with respect to the aldehyde and first order with respect to the base. The result of the Cannizzaro reaction is the self oxidation and reduction of the carbonyl compound and hence it can be classified as a disproportionation reaction.
A reddish brown gas, obtained on heating an inorganic compound with K2Cr2O7 and conc. H2SO4 was bubbled through dil. NaOH. The alkaline solution yielded a yellow precipitate on addition of lead acetate. What anion is present in the inorganic compound?
Use in Chemistry: a- halo esters and carbonyl compounds condense in presence of a base produce a, b - epoxy esters that on decarboxylation gives substituted carbonyl compounds
The reaction begins with the abstraction of the acidic hydrogen from the ester. The carbanion formed then acts as a nucleophile and gets added to the carbonyl compound to form a tetrahedral intermediate. In the next step an intramolecular nucleophilic substitution takes place in which the halide ion is eliminated to form the epoxide.
Curcuminoids are natural phenols that are responsible for the yellow colour of turmeric. Turmeric is used as a spice and belongs to the ginger family. A lot of research going on in Curcumin prove that it may be useful in prevention and also in treatment of several diseases. Presently turmeric is being investigated for possible benefits in Alzheimer’s disease, cancer and arthritis.
Curcumin can exist in a couple of tautomeric forms. It has a 1,3 diketo group which can form two equivalent enol forms. It is found that the enol form is more stable in its solid state.
The IUPAC Name of Curcumin is (1E,6E)-1,7-Bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione
Paracetamol is a common analgesic and antipyretic drug. It can be purchased over the counter of any medical store without a prescription from a doctor. It is harmless when used at recommended doses (up to 4g / day for adults) and may cause severe damages to liver when consumed in quantities more than recommended. That is why it is not recommended to consume paracetamol without the advise of a doctor.
The molecule shown below is known as acetaminophen and also as paracetamol. Both of the names are derived from its common chemical name para-acetylaminophenol. Its IUPAC Name is N-(4-hydroxyphenyl)acetamide.
Paracetamol can be prepared by nitrating phenol with a mixture of dilute H2SO4 and NaNO3. The nitrogroup is then reduced to amino group and then acylated with acetic anhydride. The scheme of preparation is shown below.
“There’s something satisfying, I think, about the idea of light traveling for millions of years through space and just at the right moment as it reaches Earth someone looks at the right bit of sky and sees it. It just seems right that an event of that magnitude should be witnessed.”—Bill Bryson (A Short History of Nearly Everything)