Substitution Reactions in Alkanes
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Questions and Answers

What is the outcome when ketones are tested with Fehling's solution?

No red precipitate is observed.

What is the result of adding ethanal to Tollen's reagent and warming it?

A silver mirror may form on the test tube.

What observable reaction occurs when magnesium is added to water?

Fizzing occurs and a mild pop is heard.

What is formed when two methyl free radicals combine?

<p>Ethane is formed.</p> Signup and view all the answers

What happens when anhydrous sodium carbonate is introduced into a test tube with acid?

<p>The lighted taper is extinguished and limewater turns milky white.</p> Signup and view all the answers

In the synthesis of PVC from ethene, what is produced when ethene reacts with chlorine?

<p>1,2-dichloroethane is produced.</p> Signup and view all the answers

What role does sulfuric acid play in the esterification process?

<p>Sulfuric acid acts as a catalyst.</p> Signup and view all the answers

How does the presence of inhibitors like oxygen affect the chain reaction?

<p>Inhibitors such as oxygen slow down the reaction.</p> Signup and view all the answers

What does the term 'vaporisation' refer to in mass spectrometry?

<p>It refers to turning the sample material into gas.</p> Signup and view all the answers

What occurs in an addition reaction?

<p>Two or more molecules react together to form a single molecule.</p> Signup and view all the answers

What is the significance of acceleration in mass spectrometry?

<p>Positive ions are attracted to a negatively charged plate and travel at high speed.</p> Signup and view all the answers

What happens during the hydrogenation of vegetable oils?

<p>Hydrogen is added across double bonds in the oils.</p> Signup and view all the answers

How does mass spectrometry separate particles?

<p>Particles are deflected by a magnetic field according to their masses.</p> Signup and view all the answers

What is the significance of polarisation in the reaction between ethene and bromine?

<p>Polarisation causes the electrons in the C=C bond to be repelled, leading to the formation of charged ions.</p> Signup and view all the answers

What is formed as a result of heterolytic fission of Br2?

<p>Br^+ and Br^- ions are formed.</p> Signup and view all the answers

What should be done after filtering the precipitated soap?

<p>Test the soap for its lathering qualities.</p> Signup and view all the answers

What is a substitution reaction and how does it apply to alkanes?

<p>A substitution reaction is when an atom or group in a molecule is replaced by another. In alkanes, such reactions occur mainly with halogens in the presence of ultraviolet light.</p> Signup and view all the answers

Describe the initiation step in the monochlorination of methane.

<p>The initiation step involves breaking down a chlorine molecule into two chlorine atoms under ultraviolet light.</p> Signup and view all the answers

What occurs during the propagation steps of the monochlorination reaction?

<p>In the first propagation step, a chlorine atom attacks a methane molecule to form hydrogen chloride and a methyl free radical; in the second step, the methyl free radical reacts with a chlorine molecule to form chloromethane.</p> Signup and view all the answers

Explain what is meant by a chain reaction in the context of the monochlorination of methane.

<p>A chain reaction is when products from one step of the reaction serve as reactants in subsequent steps, allowing the reaction to continue.</p> Signup and view all the answers

Identify the role of ultraviolet light in the halogenation of alkanes.

<p>Ultraviolet light facilitates the breakdown of chlorine molecules into chlorine radicals, which initiate the substitution reaction.</p> Signup and view all the answers

What products result from excess chlorine being used in the halogenation of methane?

<p>Excess chlorine leads to the formation of dichloromethane (CH2Cl2), trichloromethane (CHCl3), and tetrachloromethane (CCl4).</p> Signup and view all the answers

What evidence suggests a free-radical mechanism occurs during the halogenation of alkanes?

<p>The observation that reactions occur upon exposure to UV light suggests the formation of Cl radicals via the breakdown of Cl2.</p> Signup and view all the answers

What type of particles are deflected just the right amount in a spectrometer?

<p>Particles with a specific mass are deflected just the right amount.</p> Signup and view all the answers

How does the height of a peak in a mass spectrum relate to particle abundance?

<p>The height of a peak indicates the relative abundance of particles; taller peaks signify more particles.</p> Signup and view all the answers

What is the primary use of mass spectrometry in drug testing?

<p>Mass spectrometry is used for detecting and analyzing drugs within samples.</p> Signup and view all the answers

Describe the stationary phase in paper chromatography.

<p>The stationary phase in paper chromatography is chromatography paper.</p> Signup and view all the answers

What occurs during the elution process in column chromatography?

<p>During elution, a solvent passes through a column, separating the mixture components.</p> Signup and view all the answers

What is the main principle behind gas chromatography?

<p>Gas chromatography separates components based on their interactions with a solid stationary phase and a gaseous mobile phase.</p> Signup and view all the answers

What determines how far each component moves in paper chromatography?

<p>The solubility of each component in the mobile phase determines how far it moves.</p> Signup and view all the answers

List one application of paper chromatography.

<p>One application of paper chromatography is the analysis of food colorings.</p> Signup and view all the answers

What test could confirm that a colourless liquid is ethanoic acid and not ethanol?

<p>Adding acidified potassium permanganate will show no colour change.</p> Signup and view all the answers

How can ethene gas be demonstrated to be unsaturated?

<p>By adding bromine solution, the colour changes from brown to colourless.</p> Signup and view all the answers

Describe how chromatography can be used to separate a mixture of indicators.

<p>Use paper chromatography by applying the mixture on paper above the eluent; the solvent will move up and separate components.</p> Signup and view all the answers

Which benzoic acid sample is purer based on given melting points?

<p>Sample B is purer because it has a narrower melting point range.</p> Signup and view all the answers

What is the suitable solvent for recrystallizing impure benzoic acid, and why?

<p>Water is suitable because benzoic acid is very soluble in hot water but only slightly soluble in cold water.</p> Signup and view all the answers

What are the roles of the stationary and mobile phases in gas chromatography?

<p>The stationary phase is the coiled column filled with coated silica gel, while the mobile phase is an inert carrier gas that carries the samples through the column.</p> Signup and view all the answers

How does high pressure affect the mobile phase in high performance liquid chromatography (HPLC)?

<p>High pressure is necessary to push the tightly packed stationary phase, allowing for faster elution of the liquid solvent.</p> Signup and view all the answers

Why is mass spectrometry often paired with gas chromatography?

<p>Mass spectrometry is used after gas chromatography to identify each component separated from the mixture by analyzing the mass-to-charge ratio.</p> Signup and view all the answers

What type of information can be derived from an infrared (IR) spectrum of an organic compound?

<p>The IR spectrum provides a fingerprint that indicates which functional groups are present in the compound.</p> Signup and view all the answers

What is a primary use of ultraviolet (UV) spectroscopy compared to infrared (IR) spectroscopy?

<p>UV spectroscopy can measure both the absorbance of compounds and their concentrations, making it useful for quantitative analysis.</p> Signup and view all the answers

What safety or procedural step is important when setting up a chromatography experiment?

<p>It is crucial to cover the chromatography tank and allow it to stand for a few hours to saturate the tank with solvent vapor.</p> Signup and view all the answers

In chromatographic analysis, what is the significance of making a pencil line on chromatography paper?

<p>The pencil line acts as a baseline for applying the spots of indicators and helps track the solvent front during the experiment.</p> Signup and view all the answers

List two applications of gas chromatography mentioned in the content.

<p>Gas chromatography is used for measuring alcohol levels in urine and drug testing in athletes.</p> Signup and view all the answers

Study Notes

Substitution Reactions

  • A substitution reaction is a chemical reaction where an atom or group of atoms in a molecule is replaced by another atom or group.
  • Alkanes undergo substitution reactions with halogens in the presence of ultraviolet light.
  • Example: CH4 + Cl2 → CH3Cl + HCl (methane + chlorine → chloromethane + hydrogen chloride)
  • This reaction replaces a hydrogen atom in methane with a chlorine atom.
  • Further substitutions can occur if excess chlorine is used, forming dichloromethane, trichloromethane, and tetrachloromethane.

Mechanism of Monochlorination of Methane

  • The mechanism of a reaction is a detailed step-by-step description of how the overall reaction occurs.
  • The mechanism for the substitution of methane with chlorine is called free-radical substitution.
  • Initiation involves a chlorine molecule being broken down into two chlorine atoms by ultraviolet light.
  • Propagation steps involve a chlorine atom attacking a methane molecule to form hydrogen chloride and a methyl free radical. A methyl free radical then attacks a chlorine molecule to form chloromethane and a chlorine atom.
  • Termination involves the combination of chlorine radicals and methyl radicals to form Cl2, chloromethane, and ethane.

Addition Reactions

  • An addition reaction is one where two or more molecules combine to form a single molecule.
  • Addition reactions typically occur on double or triple bonds, changing the geometry from planar to tetrahedral.
  • Example: hydrogenation of vegetable oils
  • Also used to form plastics.

Evidence for Mechanism

  • Reaction takes place when exposed to UV light.
  • Thousands of chloromethane molecules are formed for every photon of light absorbed.
  • Ethane is found amongst the products.

Polymerization Reactions

  • Polymers are long-chain molecules formed by joining many small molecules (monomers).
  • Polymers form repeating structures.
  • Example: Polychloroethene (PVC)
  • The repeating unit in a polymer produces the complete polymer chain except for the end groups.
  • Example: Polyvinyl chloride from ethene

Elimination Reactions

  • An elimination reaction removes a small molecule from a larger molecule, leaving a double bond in the larger molecule.

Redox Reactions

  • Oxidation reaction of a primary alcohol using an oxidizing agent such as sodium dichromate forms an aldehyde.
  • Oxidation reaction of secondary alcohol using an oxidizing agent form a ketone.
  • Aldehydes and ketones can be reduced back to alcohols in the presence of hydrogen and a nickel catalyst.

Mandatory Experiments

  • There are several experiments on soap making, ethanol reactions, and oxidation reactions using various reagents. Instructions and procedures are included in these sections.

Mass Spectrometry

  • Mass spectrometry separates charged particles based on their masses in a magnetic field.
  • This process involves five stages: ionisation, vaporisation, acceleration, separation, and detection.
  • A mass spectrum shows the relative abundance of particles of different masses.

Chromatography

  • Chromatography separates components of a mixture based on their interactions with a mobile phase and a stationary phase.
  • Types include paper chromatography and column chromatography.
  • Paper chromatography uses paper as the stationary phase and a solvent as the mobile phase.
  • This method separates components based on their solubility in the solvent.

Organic Compounds that Act as Acids

  • The hydrogen on the -OH group of an alcohol can act as an acid when reacting with sodium, forming the sodium salt of the alcohol and hydrogen gas.
  • Carboxylic acids have slightly positive carbon atoms that pull electrons from the oxygen atoms of the -OH group, creating an inductive effect, making them acidic.
  • Carboxylate ions are more stable than many other negative ions due to delocalisation of the negative charge.

Exam Questions

  • Exam questions about the preparation of soap, different reactions, and procedures are included.

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Types of Reactions in Organic Chemistry PDF

Description

This quiz covers the concept of substitution reactions, particularly focusing on alkanes and their reactions with halogens under ultraviolet light. The mechanism of monochlorination of methane, including initiation and propagation steps, is also discussed. Test your knowledge on this key topic in organic chemistry!

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