Umm Al-Qura University Chemistry Final Exam PDF

# Umm Al-Qura University ## Faculty of Applied Science ## Department of Chemistry ### Final Exam (3rd Trimester 1444 H) **Course Title:** Physical Organic Chemistry and Stereochemistry **Course Code:** 4023551-3 **Exam Total Grade:** /50 **Allowed Time:** 10:15-12:15 (2 hours) **Student ID:** **Stud...

# Umm Al-Qura University ## Faculty of Applied Science ## Department of Chemistry ### Final Exam (3rd Trimester 1444 H) **Course Title:** Physical Organic Chemistry and Stereochemistry **Course Code:** 4023551-3 **Exam Total Grade:** /50 **Allowed Time:** 10:15-12:15 (2 hours) **Student ID:** **Student Name:** ## Answer the following questions: **(20 Marks)** **A- Put mark (x) or (√) against a statement given below:** | Statement | True / False | |:---|:---| | The intermediates formed during the reactions: short lived and mostly non isolable | √ | | Higher (ε) solvent solvate and not stabilise the carbocation more easily than lower (ε) solvents | x | | The change in entropy (ΔS) during the course of a reaction is favorable (Δ S < 0) if the number of molecules decreases | x | | Stereoisomers have the same functional groups and connectivities, they differ only in the arrangement of atoms and bonds in space | √ | | The chair form is expected to be more stable than the boat form because it has less strain between the hydrogens | √ | | Electrophiles are the electron-deficient species that tend to accept electrons | √ | | At higher temperatures particles have more energy and therefore collide more often and more effectively | √ | | A Fischer projection is a convenient two-dimensional drawing that represents a three-dimensional molecule | √ | | SN1 lead to a mixture of stereoisomers | √ | | The staggered conformation in Ethane is more stable by 4.9 kcal/mol than the eclipsed conformation | x | | The transition state exists at the point of minimum potential energy | x | | A Newman Projection can be drawn such that the groups on the front carbon are staggered (80 degrees apart) | x | | Geometric isomers are compounds that have different configurations because of the presence of rigid structure in the molecule | √ | | The chiral centre is an atom connected to five different substituent groups | x | | Optical activity is the ability of a chiral molecule to rotate the plane of plane-polarized light | √ | | The maximum number of stereoisomers that molecule can have is 2^n, where n is the number of chiral centres | x | | Endergonic and exergonic are two types of chemical reactions or processes in thermochemistry | √ | | The gauche form in butane is less stable than the anti form by 0.8 kcal/mol due to steric hindrance | x | | A Lower concentration of reactant particles allows a greater number of collisions | x | ## B- Choose One correct answer for each sentence: **(20 Marks)** 1. Stability degree of conformational isomeris of butane: * (a) Anti >Gauche >Eclipsed >Syn * (b) Gauche > Anti > Eclipsed >Syn * **(c) Anti >Eclipsed > Gauche > Syn** * (d) Syn >Gauche >Eclipsed >Anti 2. Solvent is one in which there is hydrogen capable of forming a hydrogen bond to the nucleophile called: * **(a) A protic solvent** * (b) A non-protic solvent * (c) Strong solvent * (d) Normal solvent 3. In higher polarity solvents the nucleophile is more solvated and more stable, and therefore: * (a) More reactive * **(b) Normal reactive** * (c) None reactive * (d) Less reactive 4. Stability order of alkyl carbocation is: * **(a) 3°>2°>1°> methyl** * (b) 3°>1°>2°> methyl * (c) methyl > 2°>1°>3° * (d) methyl > 1°>2°>3° 5. A catalyst provides an alternative reaction pathway that has a ____ than the uncatalyzed reaction: * **(a) lower total activation energy** * (b) higher half-life (t1/2) * (c) higher total activation energy * (d) None 6. In Decalin, the trans isomer is more stable than the cis isomer by: * **(a) 2.7 kcal/mol** * (b) 4.2 kcal/mol * (c) 5.2 kcal/mol * (d) 6.3 kcal/mol 7. The molecule 2,3-dichlorobutane has: * **(a) Two stereoisomers** * (b) Three stereoisomers * (c) Four stereoisomers * (d) Five stereoisomers 8. The rate of reaction depends on the concentrations of both reactants follow mechanism of: * **(a) SN1 Reactions** * (b) SN2 Reactions * (c) E1SN Reactions * (d) E2SN Reactions 9. A measure of the energy content of a compound is: * **(a) Gibbs free energy** * (b) Enthalpy * (c) Entropy * (d) Endergonic 10. The most stable _______ is the major product of the reaction for both E1 and reaction: * (a) Alkyl halides * (b) Alcohol * **(c) epoxy** * (d) Alkene 11. An exergonic reaction: * (a) single bond * **(b) ΔG negative** * (c) triple bond * (d) (a) and (b) 12. Aryne intermediate is an aromatic ring that contains a: * (a) double bond * **(b) triple bond** * (c) single bond * (d) None 13. A protic solvent is one in which there is hydrogen capable of forming a _______ bond to the nucleophile: * (a) pi * (b) single * **(c) sigma** * (d) Hydrogen 14. For a _______ reaction, t1/2 does not depend on the initial concentration: * **(a) Zero-order** * (b) First-order * (c) Second-order * (d) None 15. Carbenes have been isolated only by entrapment in matrices at low temperature: * (a) ≤273 K * **(b) ≤77 K** * (c) ≤177 K * (d) ≤ 298 K 16. E_a can be calculated from the ________ equation: * **(a) Arrhenius** * (b) Hammett * (c) Bronsted * (d) Taft 17. Homolytic Cleavage to form free radicals is initiated using: * (a) catalyst * **(b) light** * (c) solvent * (d) surfactant 18. In halogenations process, _______ is so exothermic that it is impractical: * **(a) Chlorination** * (b) Bromination * (c) Iodination * (d) Fluorination 19. The greater the number of ______ substituent the more stable is the alkene: * (a) Hydrogen * (b) Halogen * (c) Hydroxide * **(d) Alkyl** 20. DMSO(MeSOMe) is classified as _______ solvent: * (a) Protic and polar * **(b) Non-protic and polar** * (c) Non-protic and non-polar * (d) None ## C- Draw 1,4-dimehtylcyclohexane conformations with chair form and decide the most stable form? **(2 Marks)** **Cis** * (1,4a,4a) * (1,4e,4e) * (1,4a,4e) **Trans** * (1,4a,4d) * (1,4e,4d) * (1,4e,4e) **(1,4e,4e) is more stable** ## D- Select the absolute configuration of a chiral center in a Fisher projection by system (R), (S)? **(3 Marks)** * **1st Chiral center: R** * **2nd Chiral center: S** ## E-Select (Z), (E) system for this compounds? **(2 Marks)** * **1st Compound: Z** * **2nd Compound: Z** * **3rd Compound: E** ## F- Draw a conformational isomers of Butane (C4H10) and decide the degree of stability? **(3 Marks)** **Conformations:** | Degree | Diagram | |---|---| | 180 degree | `H` `CH3` <br> `H` `CH3` <br> `CH3` `H` <br> `H` `H`| | 240 degree | `H` `CH3` <br> `CH3` `H` <br> `H` `CH3` <br> `H` `H`| | 300 degree | `CH3` `H` <br> `H` `CH3` <br> `H` `CH3` <br> `H` `H`| | 360 degree | `H` `H` <br> `CH3` `CH3` <br> `H` `H` <br> `CH3` `CH3`| | 60 degree | `CH3` `H` <br> `H` `H` <br> `CH3` `H` <br> `H` `CH3`| | 120 degree | `CH3` `H` <br> `CH3` `H` <br> `H` `H` <br> `H` `CH3`| **Stability (Increasing order):** * 120 degree * 60 degree * 300 degree * 240 degree * 180 degree * 360 degree