Reactivity 3.3 Flashcards

A chemical species (atom, ion, or molecule) that contains one or more unpaired valence electrons, represented using a single bold dot (e.g. Cl•, •CH3).

The property of being attracted into an external magnetic field, caused by the presence of one or more unpaired electrons whose magnetic dipole moments do not cancel.

Diamagnetic substances (all electrons paired) are weakly repelled by a magnetic field. Paramagnetic substances (unpaired electrons present) are attracted into a magnetic field.

Carbon forms 3 single C-H covalent bonds and has 1 unpaired electron (7 valence electrons total). It has sp2 hybridisation and a trigonal planar geometry (~120 deg bond angles).

Nitrogen monoxide (NO, 11 valence electrons) and nitrogen dioxide (NO2, 17 valence electrons).

The breaking of a covalent bond where each bonded atom retains one of the two shared electrons, generating two electrically neutral free radicals (\(\text{X-Y} \rightarrow \text{X}^\bullet + \text{Y}^\bullet\)).

Homolytic fission breaks a bond symmetrically producing 2 neutral radicals (favoured by non-polar bonds and UV light). Heterolytic fission breaks a bond asymmetrically producing a cation and an anion (favoured by polar bonds and polar solvents).

A double-barbed curly arrow represents the movement of an electron pair (2 electrons). A single-barbed fishhook arrow represents the movement of a single electron.

Two single-barbed fishhook arrows start from the centre of the covalent bond and point in opposite directions to each bonded atom.

Energy is required to overcome the electrostatic attraction between the shared electron pair and the positively charged nuclei of the bonded atoms.

\(\lambda_{\max} = \frac{hcN_{\text{A}}}{E(\text{bond})}\), where \(h\) is Planck constant, \(c\) is speed of light, \(N_{\text{A}}\) is Avogadro constant, and \(E(\text{bond})\) is bond dissociation enthalpy in J mol⁻¹.

They contain only strong, non-polar C-C bonds (346 kJ mol⁻¹) and C-H bonds (414 kJ mol⁻¹) with no lone pairs or polar sites to attract electrophiles or nucleophiles.

1. Initiation (homolytic cleavage producing radicals). 2. Propagation (chain reactions where radicals react with molecules). 3. Termination (radicals combine to end the chain).

\(\text{Cl}_2 \xrightarrow{h\nu} 2\text{Cl}^\bullet\) (photochemical homolytic fission).

Step 1: \(\text{Cl}^\bullet + \text{CH}_4 \rightarrow {}^\bullet\text{CH}_3 + \text{HCl}\). Step 2: \({}^\bullet\text{CH}_3 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet\).

CH₄ + Cl₂ → CH₃Cl + HCl.

1. \(\text{Cl}^\bullet + \text{Cl}^\bullet \rightarrow \text{Cl}_2\). 2. \({}^\bullet\text{CH}_3 + \text{Cl}^\bullet \rightarrow \text{CH}_3\text{Cl}\). 3. \({}^\bullet\text{CH}_3 + {}^\bullet\text{CH}_3 \rightarrow \text{C}_2\text{H}_6\) (ethane).

Ethane (C2H6) can only form when two single-carbon methyl radicals (•CH3) collide and combine in a termination step, proving methyl radicals are present in the mixture.

Chloromethane product reacts further with chlorine radicals to form multi-substituted products: CH2Cl2 (dichloromethane), CHCl3 (trichloromethane), and CCl4 (tetrachloromethane).

By using a large excess of methane (CH4 >> Cl2), making collisions between Cl• and CH4 far more probable than collisions with CH3Cl.

1. \(\text{O}_2 \xrightarrow{h\nu\ (\lambda < 242\text{ nm})} 2\text{O}^\bullet\). 2. \(\text{O}^\bullet + \text{O}_2 \rightarrow \text{O}_3\). 3. \(\text{O}_3 \xrightarrow{h\nu\ (\lambda < 330\text{ nm})} \text{O}_2 + \text{O}^\bullet\). 4. \(\text{O}_3 + \text{O}^\bullet \rightarrow 2\text{O}_2\).

The C-Cl bond (338 kJ mol⁻¹) is significantly weaker than the C-F bond (484 kJ mol⁻¹). UV radiation has sufficient photon energy to cleave C-Cl but not C-F.

Step 1: \(\text{Cl}^\bullet + \text{O}_3 \rightarrow \text{ClO}^\bullet + \text{O}_2\). Step 2: \(\text{ClO}^\bullet + \text{O}^\bullet \rightarrow \text{Cl}^\bullet + \text{O}_2\). Net: \(\text{O}_3 + \text{O}^\bullet \rightarrow 2\text{O}_2\).

It operates in the same gas phase as the reactants, is consumed in the first step, and is regenerated in the second step (one Cl• can destroy up to 100,000 O3 molecules).

Step 1: \(\text{NO}^\bullet + \text{O}_3 \rightarrow \text{NO}_2^\bullet + \text{O}_2\). Step 2: \(\text{NO}_2^\bullet + \text{O}^\bullet \rightarrow \text{NO}^\bullet + \text{O}_2\). Net: \(\text{O}_3 + \text{O}^\bullet \rightarrow 2\text{O}_2\).

They contain only C, H, and F atoms (no chlorine or bromine), so they cannot produce ozone-depleting chlorine radicals (ODP = 0).

HFCs are potent greenhouse gases with high Global Warming Potential (GWP), which led to the Kigali Amendment phasing them down.