CHEM219 /CHEM 219 Module 4: (Latest Update 2026 / 2027) Principles of Organic Chemistry with Lab

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CHEM219 / CHEM 219 Module 4: (Latest Update 2026 / 2027) Principles of Organic Chemistry with Lab | Questions & Answers | Grade A | 100% Correct – Po ... rtage Learning 2026 / 2027 Academic Year Q: Describe bonding in alkanes ? - Each carbon atom is joined to 4 other atoms by single covalent bonds. - These type of covalent bonds are sigma bonds. A sigma bond results from the overlapping of 2 orbitals from each of the bonding atoms. - Each overlapping orbital has 1 electron, so the sigma bond contains 2 electrons which are shared equally between the bonding atoms. Answer - Each carbon atom has 4 sigma bonds ,( either c-c or c-h bonds ). Q: Describe the shape of alkanes ? - Each carbon atom surrounded by 4 electron pairs - electron pairs repel equally = a tetrahedral shape and bond angle of 109.5 degrees. Answer - The sigma bonds also act as an axes for which the carbon atoms can rotate freely = meaning alkane shape is not rigid and can rotate. Q: Why is fractional distillation possible ? Answer Because the boiling points of alkanes are different - they increase as their chain length increases. Q: London forces act between molecules that are in what ? Answer That are in close surface contact. Q: Effect of chain length on boiling point ? Answer - As the chain length increases, the molecules have a larger surface area = more surface contact between the molecules. - The London forces are greater = more energy required to overcome these forces. Q: The effect of branching on boiling points ? Answer - There are fewer surface points of contact on branched alkanes = fewer London forces = less energy required to overcome these forces. - Additionally, the branches get in the way and prevent the branched molecules to get as close as straight-chained molecules. - This further reduces intermolecular forces. Q: Describe the reactivity of alkanes ? Answer Alkanes do not react with most reagents. Their low reactivity is because their C-C and C-H sigma bonds are strong, and the C-C bonds are non-polar. - The electronegativity between carbon and hydrogen is so similar that C-H bonds can also be considered non-polar. Q: What do alkanes produce during combustion ? Answer They produce CO2 and water, when burnt with a plentiful supply of oxygen. Q: What produces in incomplete combustion ? Answer CO or C ( each with H20 always ) - It occurs only in UV light - its a substitution reaction - as a H atom in the alkane is substituted by a halogen atom. Q: Where does incomplete combustion take place ? Answer Takes place in a closed space = e.g. CAR ENGINE, FAULTY HEATING SYSTEM. Q: Describe the reaction of alkanes with halogens ? Answer Q: Outline first step of radical substitution for the bromination of alkanes : Include reactions : 1. Initiation : Answer When the covalent bond in a bromine molecule breaks down forming 2 bromide radicals under UV light. Br2 --> Br. + Br. Q: Outline the second step of radical substitution for the bromination of alkanes : Include the 2 reactions : Answer 2. Propagation Step 1 : Br. + CH4 --> CH3. + HBr Step 2 : CH3. + Br2 --> CH3Br + Br. - the new bromine radical then reacts with more CH4 molecules - as a CHAIN REACTION. Q: Outline the third step of radical substitution for the bromination of alkanes : Include the 3 reactions : Answer 3. Termination 2 radical collide forming a new molecules with all pairs of electrons. - meaning both radicals are removed from the reaction. Br. + Br. --> Br2 .CH3 + .CH3 --> C2H6 Br. + .CH3 --> CH3Br Q: Explain the limitations of radical substitution in organic synthesis : 1. - Further substitution : - Bromomethane can reacts with another Br. radical - substituting a further H atom to form CH2Br2 - further substitution can continue until all the Hydrogen atoms are gone leaving a mixture of CH3br , CH2Br2 , CHBr3 , CBr4 2. Substitution at different points on a carbon chain : Answer - For longer carbon chained haloalkanes - there will be a mixture of mono-substituted isomers by substitution at different points in the carbon chain. - e.g. pentane will have 3 mono substituted isomers. Q: Organohalides Answer Organic molecules contains Halogen atoms. Ex. Algae, Mollusk, sponges, can be industrially used - solvents, insecticides, herbicides, cleaning fluids, fire retardants, refrigerants. Can also be used as inhaled anesthetic. More important in organic chemistry because of theur reactivity. Q: Two of the most important reaction types typical of organic Halides are.... Answer Substitution Elimination Q: Substitution Reaction Answer Replacing the halogen in a molecule with another atom or functional group. Q: Elimination Answer Removes the halogen and hydrogen atoms from the original substrate molecule creating an unsaturated compound. Q: Alkyl halide Answer Aliphatic hydrocarbons that possess one or more halogen atom substituents. Q: Aryl Halides Answer Aromatic hydrocarbons (benzene rings) with one or more halogen atoms attached to the ring. Q: Steric Bulk Answer Structural substitution surrounding any carbon atom that bears a halogen. Methyl, primary, secondary, tertiary. Q: Primary Alkyl Halide Answer Only on e carbon is directly attached to the carbon bearing the halogen. Q: Secondary Alkyl Halid Answer Two carbons directly attached to the carbon ring bearing the halogen Q: Tertiary Alkyl Halide Answer Three carbons are directly attached to the carbon bearing the halogen Q: Methyl halide Answer Only hydrogen atoms attached to the carbon bearing the halogen Q: X Answer Symbol for generic halogen Carbon will be electron-deficient and thus hold a slight positive charge and slight negative charge on the halogen Q: Polarization of C-H bond Answer Nucleophilic substitution reaction Answer Halogen is replaced (substituted by) a new atom or functional group. Abbreviated Sn Functional Group Interconversion Answer When one type of functional group (alkyl halide) is converted to another by substitution. Substrate Answer Molecule on which the substitution occurs. Leaving group Answer Atom or group of atoms that is replaced by another when substitution occurs Nucleophile Answer A chemical species that seeks positive charge due to the presence of one or more lone pairs present on an atom in the nucleophile Typical Nucleophiles - O,N,S Nucleo = nucleus/positive charge Phile = love There can be negatively charged nucleophiles (more reactive) or neutral nucleophiles. Generic presentation of nucleophile substitution reaction Answer Nucleophile: + Substrate-Leaving group —> nucleophile-substrate + leaving group Unreactive in nucleophilic substitution reactions Answer Aryl and vinyl halides. Substrates in which the halogen is attached to a C that is a part of a multiple bond. Mechanism Answer Term used by organic chemists to describe and explain how reactions happen at the molecular level. Also called "roadmaps" because they describe the stepwise formation of products from the reactant molecules. Two Main Types of Nucleophilic Substitution REactions Answer Sn1 and Sn2 Sn2 -single step (concerted) -nucleophile attacks from the backside of the C-L bond. Nucleophile uses a lone pair of electrons to begin making a bond to the carbon. Simultaneously the bond between the C and the L begins to break. "Backside attack" -Transition state: nucleophile and leaving group are both partly attached to substrate. -L departs from the substrate with the pair of electrons from the C-L bond and nucleophile is fully bonded to the C -Inversion of the tetrahedral geometry occurs. *increasing the concentration of either the nucleophile or substrate will increase the rate of the reaction *2 is used becuae nucleophile and substrate are involved in the only step (key step) *methyl and primary alkyl halide react this way more easily FActors that influence the mechanistic path... of Sn2 -Strength of the nucleophile - SN2 depends on a strong Nucleophile, the more negative, the stronger. -Size of nucleophile - larger are more efficient -Electronegativity of the Nucleophilic atom - the more electronegative, the less nucleophilic -Solvent chosen - they are polar and require polar solvents ; best solvents are polar aprotic solvents Sn1 -two steps -Step 1 - Leaving group leaves first. C-L bond breaks and produces 2 ionic products: carbocation intermediate and a leaving group anion (this step is energy intensive and very slow) known as the rate determining step (RDS) -Step 2 - nucleophile attacks the carbocation intermediate, using a lone pair to form a covalent bond. *no inversion of tetrahedral geometry *produces 50:50 enantiomers. *secondary and tertiary typically react this way Factors that influence the mechanistic path of... Sn1 -nucleophile size, strength , concentration and electronegativity have no effect -solvent chosen - polar protic solvents - they can donate H bonds becuase they have -OH, NH, or -SH froup as part of their structure. Elimination reaction When the structure of the alkyl halide substrate contains one or more hydrogen atoms attached to the carbon adjacent to the carbon bearing the leaving group. Halogen atom is removed, along with H atom from the adjacent C atom to create an alkene. Product - ALKENE -nucleophile acts as a base and removes a H (proton) from a C adjacent to the C bearing the halogen , this C is known as the Beta C and the H associated are called Beta Hydrogens -The C bearing the leaving grou is the alpha C and associated H are alpha H Dehydrohalogenation When the reaction occurs specifically on an alkyl halide substrate. Both H and Halogen are being eliminated from the substrate molecule. E2 -one step -biomolecular -Nucleophile acts as a base and removes the beta H from the beta C, at the same time the electrons from the C-H bond come down tot for a C=C bond and break the carbon halogen bond, releasing a halide ion. E1 -several steps -alkyl halid first undergoes dissociation from the carbocation intermediate. -once formed the carbocation intermediate undergoes elimination by the nucleophile (acting as base) removing a Beta H from a beta C to form the C=C Primary Alkyl Halide (RCH2X) Will react by an Sn2 pathway if a good nucleophile (I-, Br-, RS-, NH3, or CN-) is used. There will be increasing amounts of elimination product from an E2 pathway as the basicity of the nucleophile increases (HO-, RO-). No Sn1 or E1 pathways are possible for primary substrates. Secondary Alkyl Halide(R2CHX) While all four mechanistic pathways are possible, typically these substrates will react by both Sn2 (when weak bases are used) and E2 (when strong bases are used) pathways to give a mixture of substitution and elimination products. Tertiary alkyl halide (R3X) Will react by an E2 pathway when a strong and/or bulky base is used as the nucleophile. When a weak nucleophile is used (or under acidic conditions) a mixture of substitution and elimination products will form by Sn1 and E1 mechanisms. No Sn2 pathway is available for tertiary substrates.

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