MCQs

CARBOCATION REARRANGEMENTS

Hydride Shift   

GREEN (Cl) = nucleophile     BLUE (OH) = leaving group     ORANGE (H) = hydride shift proton                                                  RED(H) = remaining proton

                  

Hydration of Alkenes: Hydride Shift

In a more complex case, when alkenes undergo hydration, we also observe hydride shift. Below is the reaction of 3-methyl-1-butene with H₃O⁺ that furnishes to make 2-methyl-2-butanol
                                                                                

 

We see this mechanism below:

    Alkyl Shift

Alkyl Shift acts very similarily to that of hydride shift. Instead of the proton (H) that shifts with the nucleophile, we see an alkyl group that shifts with the nucleophile instead. The shifting group carries its electron pair with it to furnish a bond to the neighboring or adjacent carbocation.

We see alkyl shift from a secondary carbocation to tertiary carbocation in S_N1 reactions:

#2, on the other hand, we can say that it undergoes a concerted mechanism. In short, this means that everything happens in one step. This is because primary carbocations cannot be an intermediate and they are relatively difficult processes since they require higher temperatures and longer reaction times. After protonating the alcohol substrate to form the alkyloxonium ion, the water must leave at the same time as the alkyl group shifts from the adjacent carbon to skip the formation of the unstable primary carbocation.

                  

Problems

 

Answers to Practice Problems

  

ENZYME SUBSTRATE MECHANISM

Michaelis–Menten kinetics

Main article: Michaelis–Menten kinetics

Saturation curve for an enzyme showing the relation between the concentration of substrate and rate.

Single-substrate mechanism for an enzyme reaction. k₁, k_-1 and k₂ are the rate constants for the individual steps.

As enzyme-catalysed reactions are saturable, their rate of catalysis does not show a linear response to increasing substrate. If the initial rate of the reaction is measured over a range of substrate concentrations (denoted as [S]), the reaction rate (v) increases as [S] increases, as shown on the right. However, as [S] gets higher, the enzyme becomes saturated with substrate and the rate reaches V_max, the enzyme's maximum rate.
The Michaelis–Menten kinetic model of a single-substrate reaction is shown on the right. There is an initial bimolecular reaction between the enzyme E and substrate S to form the enzyme–substrate complex ES. Although the enzymatic mechanism for the unimolecular reaction can be quite complex, there is typically one rate-determining enzymatic step that allows this reaction to be modelled as a single catalytic step with an apparent unimolecular rate constant k_cat. If the reaction path proceeds over one or several intermediates, k_cat will be a function of several elementary rate constants, whereas in the simplest case of a single elementary reaction (e.g. no intermediates) it will be identical to the elementary unimolecular rate constant k₂. The apparent unimolecular rate constant k_cat is also called turnover number and denotes the maximum number of enzymatic reactions catalysed per second.
The Michaelis–Menten equation^[9] describes how the (initial) reaction rate v₀ depends on the position of the substrate-binding equilibrium and the rate constant k₂.

    (Michaelis–Menten equation)

with the constants

This Michaelis–Menten equation is the basis for most single-substrate enzyme kinetics. Two crucial assumptions underlie this equation (apart from the general assumption about the mechanism only involving no intermediate or product inhibition, and there is no allostericity or cooperativity). The first assumption is the so called quasi-steady-state assumption (or pseudo-steady-state hypothesis), namely that the concentration of the substrate-bound enzyme (and hence also the unbound enzyme) changes much more slowly than those of the product and substrate and thus the change over time of the complex can be set to zero . The second assumption is that the total enzyme concentration does not change over time, thus . A complete derivation can be found here.
The Michaelis constant K_M is experimentally defined as the concentration at which the rate of the enzyme reaction is half V_max, which can be verified by substituting [S] = K_m into the Michaelis–Menten equation and can also be seen graphically. If the rate-determining enzymatic step is slow compared to substrate dissociation (), the Michaelis constant K_M is roughly the dissociation constant K_D of the ES complex.
If [S] is small compared to K_M then the term and also very little ES complex is formed, thus . Therefore, the rate of product formation is

Thus the product formation rate depends on the enzyme concentration as well as on the substrate concentration, the equation resembles a bimolecular reaction with a corresponding pseudo-second order rate constant k₂ / K_M. This constant is a measure of catalytic efficiency. The most efficient enzymes reach a k₂ / K_M in the range of 10⁸ - 10¹⁰ M⁻¹ s⁻¹. These enzymes are so efficient they effectively catalyse a reaction each time they encounter a substrate molecule and have thus reached an upper theoretical limit for efficiency (diffusion limit); these enzymes have often been termed perfect enzymes.^[10]       

OXIDATION -REDUCTION

Oxidation and reduction in terms of electron transfer
This is easily the most important use of the terms oxidation and reduction at A' level.
Definitions

• Oxidation is loss of electrons.
• Reduction is gain of electrons.

It is essential that you remember these definitions. There is a very easy way to do this. As long as you remember that you are talking about electron transfer:

A simple example
The equation shows a simple redox reaction which can obviously be described in terms of oxygen transfer.

Copper(II) oxide and magnesium oxide are both ionic. The metals obviously aren't. If you rewrite this as an ionic equation, it turns out that the oxide ions are spectator ions and you are left with:

Standard Reduction Potential

Reduction potential is used to calculate the standard electrode potential (E^ocell).
This is the equation most commonly seen in textbooks:
E^ocell = E^o_red + E^o_ox .
where: E^ocell is the standard electrode potential (in volts).
E^o_red is standard reduction potential of the reducing agent.
E^o_ox (standard oxidation potential) is negative of the standard reduction potential of the oxidizing agent.
though the following equation is generally more useful as one is usually only given reduction potentials, not oxidation potentials:


E^ocell = E^o_red - E^o_ox .
or equivalently:
E^ocell = E^o_cathode - E^o_anode
where:
E^ocell is the standard electrode potential (in volts).
E^o_red (E^o_cathode) is standard reduction potential of the reducing agent.
 E^o_ox (E^o_anode) is the standard reduction potential of the oxidizing agent.
                                                                                                                                                                                                                                                                             

IUPAC Naming of coordination compounds

A complex is a substance in which a metal atom or ion is associated with a group of neutral molecules or anions called ligands. Coordination compounds are neutral substances (i.e. uncharged) in which at least one ion is present as a complex.

The coordination compounds are named in the following way. 

  A. To name a coordination compound, no matter whether the complex ion is the cation or the anion, Write   always name the cation before the anion. (This is just like naming an ionic compound.)                               

   B. In naming the complex ion:

1. Name the ligands first, in alphabetical order, then the metal atom or ion. Note: The metal atom or ion is written before the ligands in the chemical formula.
2. The names of some common ligands are listed in Table 1.
? For anionic ligands end in "-o"; for anions that end in "-ide"(e.g. chloride), "-ate" (e.g. sulfate, nitrate), and "-ite" (e.g. nirite), change the endings as follows: -ide -o; -ate -ato; -ite -ito
? For neutral ligands, the common name of the molecule is used e.g. H₂NCH₂CH₂NH₂ (ethylenediamine). Important exceptions: water is called ‘aqua’, ammonia is called ‘ammine’, carbon monoxide is called ‘carbonyl’, and the N₂ and O₂ are called ‘dinitrogen’ and ‘dioxygen’.

Table 1. Names of Some Common Ligands

Anionic Ligands	Names		Neutral Ligands	Names
Br-	bromo		NH3	ammine
F-	fluoro		H2O	aqua
O2-	oxo		NO	Nitrosyl
OH-	Hydroxo		CO	Carbonyl
CN-	cyano		O2	dioxygen
C2O42-	oxalato		N2	dinitrogen
CO32-	carbonato		C5H5N	pyridine
CH3COO-	acetato		H2NCH2CH2NH2	ethylenediamine

3. Greek prefixes are used to designate the number of each type of ligand in the complex ion, e.g. di-, tri- and tetra-. If the ligand already contains a Greek prefix (e.g. ethylenediamine) or if it is polydentate ligands (ie. can attach at more than one binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are used instead. (See examples 3 and 4.) The numerical prefixes are listed in Table 2.

Table 2. Numerical Prefixes

Number	Prefix	Number	Prefix	Number	Prefix
1	mono	5	penta (pentakis)	9	nona (ennea)
2	di (bis)	6	hexa (hexakis)	10	deca
3	tri (tris)	7	hepta	11	undeca
4	tetra (tetrakis)	8	octa	12	dodeca

 

4. After naming the ligands, name the central metal. If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is call cobalt and Pt is called platinum. (See examples 1-4). If the complex ion is an anion, the name of the metal ends with the suffix –ate. (See examples 5 and 6.). For example, Co in a complex anion is called cobaltate and Pt is called platinate. For some metals, the Latin names are used in the complex anions e.g. Fe is called ferrate (not ironate).

Table 3: Name of Metals in Anionic Complexes

Name of Metal	Name in an Anionic Complex
Iron	Ferrate
Copper	Cuprate
Lead	Plumbate
Silver	Argenate
Gold	Aurate
Tin	Stannate

 

5. Following the name of the metal, the oxidation state of the metal in the complex is given as a Roman numeral in parentheses.

C. To name a neutral complex molecule, follow the rules of naming a complex cation. Remember: Name the (possibly complex) cation BEFORE the (possibly complex) anion.See examples 7 and 8.
For historic reasons, some coordination compounds are called by their common names. For example, Fe(CN)₆^3- and Fe(CN)₆^4- are named ferricyanide and ferrocyanide respectively, and Fe(CO)₅ is called iron carbonyl.

Examples Give the systematic names for the following coordination compounds:

1. [Cr(NH₃)₃(H₂O)₃]Cl₃

Answer: triamminetriaquachromium(III) chloride
Solution: The complex ion is inside the parentheses, which is a cation.
The ammine ligands are named before the aqua ligands according to alphabetical order.
Since there are three chlorides binding with the complex ion, the charge on the complex ion must be +3 ( since the compound is electrically neutral).
From the charge on the complex ion and the charge on the ligands, we can calculate the oxidation number of the metal. In this example, all the ligands are neutral molecules. Therefore, the oxidation number of chromium must be same as the charge of the complex ion, +3.

2. [Pt(NH₃)₅Cl]Br₃

Answer: pentaamminechloroplatinum(IV) bromide
Solution: The complex ion is a cation, the counter anion is the 3 bromides.
The charge of the complex ion must be +3 since it bonds with 3 bromides.
The NH₃ are neutral molecules while the chloride carries - 1 charge. Therefore, the oxidation number of platinum must be +4.

3. [Pt(H₂NCH₂CH₂NH₂)₂Cl₂]Cl₂

Answer: dichlorobis(ethylenediamine)platinum(IV) chloride
Solution: ethylenediamine is a bidentate ligand, the bis- prefix is used instead of di-

4. [Co(H₂NCH₂CH₂NH₂)₃]₂(SO₄)₃

Answer: tris(ethylenediamine)cobalt(III) sulfate
Solution: The sulfate is the counter anion in this molecule. Since it takes 3 sulfates to bond with two complex cations, the charge on each complex cation must be +3.
Since ethylenediamine is a neutral molecule, the oxidation number of cobalt in the complex ion must be +3.
Again, remember that you never have to indicate the number of cations and anions in the name of an ionic compound.

5. K₄[Fe(CN)₆]

Answer: potassium hexacyanoferrate(II)
Solution: potassium is the cation and the complex ion is the anion.
Since there are 4 K⁺ binding with a complex ion, the charge on the complex ion must be - 4.
Since each ligand carries –1 charge, the oxidation number of Fe must be +2.
The common name of this compound is potassium ferrocyanide.oxalato)nickelate(II)

Solution: The oxalate ion is a bidentate ligand.

10. [Ag(NH₃)₂][Ag(CN)₂]

Answer: diamminesilver(I) dicyanoargentate(I)
You can have a compound where both the cation and the anion are complex ions. Notice how the name of the metal differs even though they are the same metal ions.

Can you give the molecular formulas of the following coordination compounds?

1. hexaammineiron(III) nitrate
2. ammonium tetrachlorocuprate(II)
3. sodium monochloropentacyanoferrate(III)
4. potassium hexafluorocobaltate(III)

Can you give the name of the following coordination compounds?

5. [CoBr(NH₃)₅]SO₄
6. [Fe(NH₃)₆][Cr(CN)₆]
7. [Co(SO₄)(NH₃)₅]⁺
8. [Fe(OH)(H₂O)₅]²⁺

Answers:

1. [Fe(NH₃)₆](NO₃)₃
2. (NH₄)₂[CuCl₄]
3. Na₃[FeCl₁(CN)₅]
4. K₃[CoF₆]
5. pentaamminebromocobalt(III) sulfate
6. hexaammineiron(III) hexacyanochromate (III)
7. pentaamminesulfatocobalt(III) ion

8. pentaaquahydroxoiron(III) ion

6. Na₂[NiCl₄]

Answer: sodium tetrachloronickelate(II)
Solution: The complex ion is the anion so we have to add the suffix –ate in the name of the metal.

7. Pt(NH₃)₂Cl₄

Answer: diamminetetrachloroplatinum(IV)
Solution: This is a neutral molecule because the charge on Pt⁺⁴ equals the negative charges on the four chloro ligands.
If the compound is [Pt(NH₃)₂Cl₂]Cl₂, eventhough the number of ions and atoms in the molecule are identical to the example, it should be named: diamminedichloroplatinum(II) chloride, a big difference.

8. Fe(CO)₅

Answer: pentacarbonyliron(0)
Solution: Since it is a neutral complex, it is named in the same way as a complex cation. The common name of this compound, iron carbonyl, is used more often.

9. (NH₄)₂[Ni(C₂O₄)₂(H₂O)₂]

Answer: ammonium diaquabis(oxalato)nickelate(II)