Chemistry Test - 11

Main Concept : 

Faraday's laws electrolysis, First law, Second law

The laws, which govern the deposition of substances (In the form of ions) on electrodes during the process of electrolysis, is called Faraday's laws of electrolysis. These laws given by Michael Faraday in 1833.

(1) Faraday's first law : It states that,

“The mass of any substance deposited or liberated at any electrode is directly proportional to the quantity of electricity passed.”i.e.,

Main Concept :
Balancing redox reactions: Oxidation number method and ion-electron methodOxidation Number Method

During a redox reaction, the total increase in oxidation number must be equal to total decrease in oxidation number. This is the basic principle for balancing chemical equations. In addition, the number of atoms of each kind on one side of the equation must be equal to the number of atoms of the corresponding elements on the other side (the law of conservation of mass should not be violated). The following steps should be followed:

Steps for balancing redox equations by oxidation number method

Write the skeleton redox reaction.

Indicate the oxidation number of atoms in each compound above the symbol of the element.

Identify the element or elements, which undergo a change in oxidation number, one whose oxidation number increases (reducing agent) and the other whose oxidation number decreases (oxidizing agent).

Calculate the increase or decrease in oxidation numbers per atom. Multiply this number of increase/decrease of oxidation number, with the number of atoms, which are undergoing change.

Equate the increase in oxidation number with decrease in oxidation number on the reactant side by multiplying the formulae of the oxidizing and reducing agents.

Balance the equation with respect to all other atoms except hydrogen and oxygen.

Finally, balance hydrogen and oxygen.

For reactions taking place in acidic solutions, add H⁺ ions to the side deficient in hydrogen atoms.

For reactions taking place in basic solutions, add H₂O molecules to the side deficient in hydrogen atoms and simultaneously add equal number to OH^- ions on the other side of the equation.

Ion-electron method (Half reaction method)

Jette and LaMev developed the method for balancing redox-reactions by ion electron method in 1927. It involves the following steps.

(i) Write down the redox reaction in ionic form.

(ii) Split the redox reaction into two half reactions, one for oxidation and other for reduction.

(iii) Balance each half reaction for the number of atoms of each element. For this purpose, ​

(a) Balance the atoms other than H and O for each half reaction using simple multiples.

(b) Add water molecules to the side deficient in oxygen and H⁺ to the side deficient in hydrogen.

This is done in acidic or neutral solutions.

(c) In alkaline solution, for each excess of oxygen, add one water molecule to the same side and 2OH^– ions to the other side. If hydrogen is still unbalanced, add one OH^– ion for each excess hydrogen on the same side and one water molecule to the other side.

(iv) Add electrons to the side deficient in electrons as to equalise the charge on both sides.

(v) Multiply one or both the half reactions by a suitable number so that number of electrons become equal in both the equations.

(v) Multiply one or both the half reactions by a suitable number so that number of electrons become equal in both the equations.

Picric acid is a yellow coloured compound whose chemical name is 2, 4, 6 - Trinitro Phenol

Main Concept :

Nitration of Phenols: Reaction of Phenols with dilute nitric acid

Only dilute acid will be required for the nitration of phenol, nitric acid contains a small amount of nitrous acid which because of the activation of the ring will be more than enough to nitrate the phenol. Two products are formed, 2-nitrophenol and 4-nitrophenol.

.

The effect of the Intramolecular bonding makes these two compounds easy to separate, steam distillation of the reaction liquid will give the 2-nitrophenol first. 

Other Concepts :

Concept 1 :
Chemical properties of Phenols

Properties of phenol as an acid

Dakin Reaction:

With indicators

The pH of a typical dilute solution of phenol in water is likely to be around 5 - 6 (depending on its concentration). That means that a very dilute solution isn't really acidic enough to turn litmus paper fully red. Litmus paper is blue at pH 8 and red at pH 5. Anything in between is going to show as some shade of "neutral".

With sodium hydroxide solution

Phenol reacts with sodium hydroxide solution to give a colourless solution containing sodium phenoxide.

In this reaction, the hydrogen ion has been removed by the strongly basic hydroxide ion in the sodium hydroxide solution.

With sodium carbonate or sodium hydrogencarbonate

Phenol is not acidic enough to react with either of these. Or, looked at another way, the carbonate and hydrogen carbonate ions aren't strong enough bases to take a hydrogen ion from the phenol.

Unlike the majority of acids, phenol doesn't give carbon dioxide when you mix it with one of these.

This lack of reaction is actually useful. You can recognize  phenol because:

•   It is fairly insoluble in water.

•   It reacts with sodium hydroxide solution to give a colourless solution (and therefore must be acidic).

•   It doesn't react with sodium carbonate or hydrogen carbonate solutions (and so must be only very weakly acidic).

With metallic sodium

Acids react with the more reactive metals to give hydrogen gas. Phenol is no exception - the only difference is the slow reaction because phenol is such a weak acid.

Phenol is warmed in a dry tube until it is molten, and a small piece of sodium added. There is some fizzing as hydrogen gas is given off. The mixture left in the tube will contain sodium phenoxide.

As r₊/r_- lies in the range 0.732 - 1.000, coordination no. 8, and this type of void formed is simple cubic crystal. i.e. it is of CsCl type.

Main Concept :
Radius ratio, Voids, Co-ordination number and 3D geometry of ionic solids

In most ionic compounds, the anions are much larger than the cations, and it is the anions which form the crystal array. The smaller cations reside in the holes between the anions.

Basic Concepts:

I. Ions are assumed to be charged, incompressible, nonpolarizable spheres.

II. Ions try to surround themselves with as many ions of opposite charge as closely as possible. Usually in the packing arrangement, the cation is just large enough to allow te anions to surround it without touching one another.

III. The cation to anion ratio must reflect the stoichiometry of the compound. For  the lattice must be an array of chloride anions with only half that number of magnesium ion.

The packing arrangement adopted by an ionic compound is determined by the comparative size of the ions. Consider a lattice in which the anions of a part of a cubic layer. The dashed circle represents the anions below and above the plane. The shaded circle shows the interstitial space available for a cation to fit between the six anions. The cation has to be the size of the shaded circle. Using geometry, we can work out the ideal radius ratio for perfect packing. Using the Pythagorean theorem, the optimum ratio of cation radius to anion radius is 

If the cation is too large to give the optimum 0.414 ratio, the anions will be forced apart. When the radius ratio exceeds 0.732, it becomes possible to fit eight anions around the cation. When the ratio is less than 0.414, the anions will be too close together, and the anions will adopt an arrangement that has smaller cavities surrounded be only four anions. 

It is possible to predict the coordination number for salts not corresponding to any of the types listed in the table above.

i) The radius ratio predict the coordination number for the less abundant ion in any lattice type and stoichiometry. This is true because the less abundant ions have more neighbors of the opposite charge so crowding is an important issue. Except for compounds such as , the less abundant ion will be the cation. Using the radius ratio values listed in section D of the table, the coordination number of the cation can be predicted.

ii) The average coordination number of the anion, the more abundant, can be determined from the stoichiometry of the salt:

Anions are usually larger than cations giving a radius ratio less than 1.00. If the ratio is greater than 1.00, the cation is the larger of the two. The salt usually adopts one of the known lattice types with the cation and anion reversing roles in the structure. In these cases, calculate the inverse ratio and add the prefix anti-to the lattice type name.
Other Concepts :

Concept 1 :
Important ionic solidsStructure of ionic crystals

Solution:

CsCl crystal structure along with coordination is show below. The Cs⁺ ion is surrounded by 8 Cl^- ions at the corners of unit cell. It is also clearly shown in the extended unit cells that the Cl^- ion is also surrounded by 8 Cs⁺ ions.

Since the reactant is a 3° alkyl halide, so in the presence of NaCN, it will follow E2 path rather than S_N2, so path (II) is not feasible.

The possible product by path (II) is:

Main Concept :
S_N2 substitution vs. E₂ elimination of haloalkanesIntroduction:

The first set of reactions and mechanisms that are commonly taught are the substitution and elimination reactions. Each of these can go by either a one-step (S_N2 or E₂) or two-step mechanism (S_N1 or E₁). So overall, there are four possible mechanisms as well as combinations of mechanisms.

The difficulty is that all four mechanisms have exactly the same reactants: an alkyl halide and a nucleophile/base. Therefore, deciding which of the four mechanisms or combination of mechanisms is operative can be very confusing.

How to: The keys to deciding the mechanism(s) is to classify the reactivities of the two reactants.

I. Classify the alkyl halide (R-X) as either: Methyl, or 3°

II. Classify the nucleophile/base as either a strong or weak nucleophile, strong or weak base, or a bulky or not bulky base. Since nucleophilicity and basicity trends are related, there are only four possible combinations.

i. Strong nucleophile and strong base

ii. Weak (bulky) nucleophile and strong base

iii. Strong nucleophile and weak base

iv. Weak nucleophile and weak base

III. Use the flowchart or table below to decide which mechanism or combination of mechanisms will be operative. Either method should give the same answer.

Comments:

I. There is one additional criteria for the E₂ mechanism. There must be an anti-beta-hydrogen. This is a hydrogen on the carbon adjacent to the carbon bearing the halide that is on the opposite side of the molecule from the halide.

II. Note that the solvent is not involved in the decision making process. However, the choice of solvent can accelerate the reaction rates of specific mechanisms.

III. Overall, the flowchart and table are vast simplifications. For example, there are cases of mixtures of S_N1 and S_N2 mechanisms. There also are cases of mixtures of SN2 and E₂ mechanisms. However, for the purposes of this class, we will stick to this limited set of combinations and outcomes

Example 1:

2-Ketopropanoic acid

Main Concept :
Naming compounds that contain more than one types of principal functional groups

If more than one types of principal groups are present, group higher in priority becomes principal of principals and forms suffix of compound name. Prefixes for the remaining groups are used. Priority of principal functional groups are :

Carboxylic acid < Sulphonic acid < Anhydride < Ester < Acid chloride < Amide < Nitrile < Aldehydes < Ketones < Alcohols < Amines < Ethers.

Examples,

Other Concepts :

Concept 1 :
IUPAC Rules for functional compounds

Naming Aldehydes

The IUPAC system of nomenclature assigns a characteristic suffix -al to aldehydes. For example, H₂C=O is methanal, more commonly called formaldehyde. Since an aldehyde carbonyl group must always lie at the end of a carbon chain, it is always is given the #1 location position in numbering and it is not necessary to include it in the name. There are several simple carbonyl containing compounds which have common names which are retained by IUPAC.

Also, there is a common method for naming aldehydes and ketones. For aldehydes common parent chain names, similar to those used for carboxylic acids, are used and the suffix -aldehyde is added to the end. In common names of aldehydes, carbon atoms near the carbonyl group are often designated by Greek letters. The atom adjacent to the carbonyl function is alpha, the next removed is beta and so on.

(-CHO) is attached to a ring the suffix -carbaldehyde is added to the name of the ring. The carbon attached to this moiety will get the #1 location number in naming the ring.

Summary of Aldehyde Nomenclature rules

I. Aldehydes take their name from their parent alkane chains. The -e is removed from the end and is replaced with -al.

II. The aldehyde funtional group is given the #1 numbering location and this number is not included in the name.

III. For the common name of aldehydes start with the common parent chain name and add the suffix -aldehyde. Substituent positions are shown with Greek letters.

IV. When the -CHO functional group is attached to a ring the suffix -carbaldehyde is added, and the carbon attached to that group is C1.

Naming Ketones

The IUPAC system of nomenclature assigns a characteristic suffix of -one to ketones. A ketone carbonyl function may be located anywhere within a chain or ring, and its position is usually given by a location number. Chain numbering normally starts from the end nearest the carbonyl group. Very simple ketones, such as propanone and phenylethanone do not require a locator number, since there is only one possible site for a ketone carbonyl function

The common names for ketones are formed by naming both alkyl groups attached to the carbonyl then adding the suffix -ketone. The attached alkyl groups are arranged in the name alphabetically.

Summary of Ketone Nomenclature rules

I. Ketones take their name from their parent alkane chains. The ending -e is removed and replaced with -one.

II. The common name for ketones are simply the substituent groups listed alphabetically + ketone.

III. Some common ketones are known by their generic names. Such as the fact that propanone is commonly referred to as acetone.Naming Aldehydes and Ketones in the Same MoleculeAs with many molecules with two or more functional groups, one is given priority while the other is named as a substituent. Because aldehydes have a higher priority than ketones, molecules which contain both functional groups are named as aldehydes and the ketone is named as an "oxo" substituent. It is not necessary to give the aldehyde functional group a location number, however, it is usually necessary to give a location number to the ketone.

Naming Dialdehydes and Diketones

For dialdehydes the location numbers for both carbonyls are omitted because the aldehyde functional groups are expected to occupy the ends of the parent chain. The ending -dial is added to the end of the parent chain name.For diketones both carbonyls require a location number. The ending -dione or -dial is added to the end of the parent chain.

Naming Cyclic Ketones and Diketones

In cyclic ketones the carbonyl group is assigned location position #1, and this number is not included in the name, unless more than one carbonyl group is present. The rest of the ring is numbered to give substituents the lowest possible location numbers. Remember the prefixcyclo is included before the parent chain name to indicate that it is in a ring. As with other ketones the -e ending is replaced with the -one to indicate the presence of a ketone.With cycloalkanes which contain two ketones both carbonyls need to be given a location numbers. Also, an -e is not removed from the end but the suffix -dione is added.

Naming Carbonyls and Hydroxyls in the Same Molecule

When and aldehyde or ketone is present in a molecule which also contains an alcohol functional group the carbonyl is given nomenclature priority by the IUPAC system. This means that the carbonyl is given the lowest possible location number and the appropriate nomenclature suffix is included. In the case of alcohols the OH is named as a hydroxyl substituent. However, the l in hydroxyl is generally removed

Naming Carbonyls and Alkenes in the Same Molecule

When and aldehyde or ketone is present in a molecule which also contains an alkene functional group the carbonyl is given nomenclature priority by the IUPAC system. This means that the carbonyl is given the lowest possible location number and the appropriate nomenclature suffix is included.When carbonyls are included with an alkene the following order is followed:(Location number of the alkene)-(Prefix name for the longest carbon chain minus the -ane ending)-(an -en ending to indicate the presence of an alkene)-(the location number of the carbonyl if a ketone is present)-(either an -one or and -anal ending). Remember that the carbonyl has priority so it should get the lowest possible location number. Also, remember that cis/tran or E/Z nomenclature for the alkene needs to be included if necessary.

When fat is heated in presence of KHSO₄ (dehydrating agent) the glycerol portion of the molecule is dehydrated and forms unsaturated aldehyde CH₂=CH— CHO (acrolein), a bad smelling compound. It is the test for fat.

KEY CONCEPTS

Natural Fats

• These are esters of fatty acids with glycerol, e.g. oil, butter, ghee, margarine, etc.
• Fats and oils are esters derived from glycerol (an alcohol) and fatty acids. Physical nature of fat is determined by the chain length of fatty acids and whether they are saturated or unsaturated.
• Most of the animals usually get the supply of neutral fats from food.
• Normally, three fatty acids can join a glycerol molecule. These are called as triglycerides (Triacyl glycerol)

Physical properties
• Triglycerides are found in solid or liquid forms depending upon the nature of the constituent fatty acid.
• Most of the plant triglycerides bear low melting points and are liquid at room temperature because they contain unsaturated fatty acid.
• Animal triglycerides contain higher quantity of saturated fatty acids and are semi-solid or solid at room temperature with high melting points.
• Neutral fats are colorless, odorless and tasteless substances.
• Due to their lower specific gravities, they float on water.

Chemical properties

• Hydrolysis - On boiling with acids or alkalies neutral fats hydrolyze to glycerol and fatty acids. Fatty acids liberated combine with base to form soaps, if alkali has been used.
• Oxidation - Fats have many unsaturated fatty acids e.g. linseed oil undergo spontaneous oxidation. This occurs at double bond and forms aldehydes, ketones and resins.
• Rancidity - Naturally occurring animal fats due to presence of enzyme lipase undergo partial hydrolysis and some degree of oxidation at the double bond of unsaturated fatty acids. Such fats develop a characteristic taste and odor and process is called rancidity.
• Additive reactions - In neutral fats, unsaturated acids exhibit reactions like hydrogenation and halogenation. Oils on hydrogenation become solidified and form Vanaspati ghee.

Condensation of NH₂ - NH₂ at both nitrogens with C₆H₅CHO gives the desired product.

KEY CONCEPTS

Hydrazine

Hydrazine forms a monohydrate that is denser than the anhydrous material.

Hydrazine can arise via coupling a pair of ammonia molecules by removal of one hydrogen per molecule. Each H₂N - N sub-unit is pyramidal in shape. The N - N single bond distance is 1.45 Å (145 pm), and the molecule adopts a gauche conformation. The rotational barrier is twice that of ethane. These structural properties resemble those of gaseous hydrogen peroxide, which adopts a "skewed" anticlinal conformation, and also experiences a strong rotational barrier.

Hydrazine has basic (alkali) chemical properties comparable to those of ammonia:

Nitrogen containing compounds

The important nitrogen containing organic compounds are alkyl nitrites (RONO), nitro-alkanes (RNO₂), aromatic nitro compounds (ArNO₂), alkyl cyanides (RCN), alkyl iso cyanides (RNC), amines b, aryl diazonium salts (ArN₂Cl), amides b and oximes

The relative rate of 3^o bridgehead system towards solvolysis can be explained using, (i) strain energy & (ii) stability of carbocation intermediate.

Main Concept :
Stability of carbocation

Structure and properties

The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability (octet rule). Therefore, carbocations are often reactive, seeking to fill the octet of valence electrons as well as regain a neutral charge. One could reasonably assume a carbocation to have sp³ hybridization with an empty sp³ orbital giving positive charge. However, the reactivity of a carbocation more closely resembles sp² hybridization with a trigonal planar molecular geometry. An example is the methyl cation, .

Order of stability of examples of tertiary (III), secondary (II), and primary (I) alkylcarbenium ions, as well as the methyl cation (far right). Carbocations are often the target of nucleophilic attack by nucleophiles like hydroxide (OH^-) ions or halogen ions.

Carbocations typically undergo rearrangement reactions from less stable structures to equally stable or more stable ones with rate constants in excess of 10⁹/sec. This fact complicates synthetic pathways to many compounds. For example, when 3-pentanol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a statistical mixture of the 3-pentyl and 2-pentyl. These cations react with chloride ion to produce about  3-chloropentane and  2-chloropentane.

A carbocation may be stabilized by resonance by a carbon-carbon double bond next to the ionized carbon. Such cations as allyl cation  and benzyl cation  are more stable than most other carbocations. Molecules that can form allyl or benzyl carbocations are especially reactive. These carbocations where the C⁺ is adjacent to another carbon atom that has a double or triple bond have extra stability because of the overlap of the empty p orbital of the carbocation with the p orbitals of the π-bond. This overlap of the orbitals allows the charge to be shared between multiple atoms – delocalization of the charge - and, therefore, stabilizes the carbocation. Hyperconjugation is also a stabilizing factor for carbocations. The empty pi orbitals of the carbon atom accepts a pair of electrons from the alpha carbon which then acquires the positive charge. More alpha hydrogens increases the stability of carbocation. Stability order also follows sp³ > sp² > sp hybridization of the carbon atom bearing positive charge

Bredt's rule - Carbocations

Bredt's rule is an empirical observation in organic chemistry that states that a double bond cannot be placed at the bridgehead of a bridged ring system, unless the rings are large enough. The rule is named after Julius Bredt. For example, two of the following isomers of norbornene violate Bredt's rule, which makes them too unstable to prepare:

In the diagram, the bridgehead atoms involved in Bredt's rule violation are highlighted in red.

Bredt's rule is a consequence of the fact that having a double bond on a bridgehead would be equivalent to having a trans double bond on a ring, which is not possible for small rings (fewer than eight atoms) due to a combination of ring strain, and angle strain (nonplanar alkene). The p-orbitals of the bridgehead atom and adjacent atoms are orthogonal and thus are not aligned properly for the formation of ππ-bonds.

Bredt's rule can be useful for predicting which isomer is obtained from an elimination reaction in a bridged ring system. It can also be applied to reaction mechanisms that go via carbocations and, to a lesser degree, via free radicals, because these intermediates, like carbon atoms involved in a double bond, prefer to have a planar geometry with 120^o angles and sp² hybridization.

An anti-bredt molecule is one that is found to exist and be stable (within certain parameters) despite this rule. A recent (2006) example of such a molecule is 2-quinuclidonium tetrafluoroborate.

Main Concept :
Unique value of reaction rate A reaction rate can be reported quite differently depending on which product or reagent selected to be monitored.

Given a reaction:

Even though the concentrations of A, B, C and D may all change at different rates, there is only one average rate of reaction. To get this unique rate, choose any one rate and divide it by the stoichiometric coefficient. When the reaction has the formula: