Introduction to Organic Chemistry: Mechanisms behind hydrohalogenation, hydration, bromination, and hydroboration
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Questions and Answers

What is the bond angle around a positively charged carbon in a carbocation?

  • 120° (correct)
  • 180°
  • 90°
  • 105°

    • Which type of carbocation is the most stable?

  • Methyl carbocation
  • 2° carbocation
  • 1° carbocation
  • 3° carbocation (correct)

    • What phenomenon explains the polarization of electron density toward a positively charged carbon?

  • Resonance effect
  • Hybridization effect
  • Inductive effect (correct)
  • Electrostatic effect

    • Which statement about methyl and 1° carbocations is true?

    <p>They are so unstable that they are not observed in reactions.</p> Signup and view all the answers

    How does the stability of carbocations relate to the area of charge delocalization?

    <p>Greater stability is associated with wider charge delocalization.</p> Signup and view all the answers

    What characteristic makes a carbocation an electrophile?

    <p>It has a positive charge and accepts electron pairs.</p> Signup and view all the answers

    Which of the following carbocations requires the lowest activation energy for formation?

    <p>3° carbocation</p> Signup and view all the answers

    In terms of hybridization, which type of orbitals are used by carbon in a carbocation?

    <p>sp2 hybrid orbitals</p> Signup and view all the answers

    What type of rearrangement occurs when a 2° carbocation shifts to form a 3° carbocation?

    <p>1,2-shift</p> Signup and view all the answers

    What is the primary product when 2-methylpropene reacts with methanol in the presence of an acid catalyst?

    <p>tert-butylmethyl ether</p> Signup and view all the answers

    Which of the following correctly describes a stereoselective reaction?

    <p>A reaction that favors the formation or destruction of one stereoisomer</p> Signup and view all the answers

    What type of ion is formed as a key intermediate during the addition of bromine to an alkene?

    <p>Cyclic bromonium ion</p> Signup and view all the answers

    In the mechanism of electrophilic addition of bromine to alkenes, what is the first step?

    <p>Formation of a bridged halonium ion</p> Signup and view all the answers

    Which statement correctly describes carbocation stability?

    <p>Secondary carbocations can rearrange to form more stable tertiary carbocations</p> Signup and view all the answers

    What is a characteristic of the addition of Cl2 and Br2 to alkenes?

    <p>It demonstrates anti stereoselectivity.</p> Signup and view all the answers

    When a positively charged intermediate forms during a carbocation reaction, which step usually follows?

    <p>Nucleophile attack to form a covalent bond</p> Signup and view all the answers

    What is a key feature of the first step in the acid-catalyzed hydration of alkenes?

    <p>Proton transfer to form a 2° carbocation</p> Signup and view all the answers

    Which statement best describes Markovnikov's Rule in relation to hydration reactions?

    <p>The more substituted carbon will receive the hydroxyl group.</p> Signup and view all the answers

    What factor primarily determines the stability of a carbocation?

    <p>The number of attached alkyl groups</p> Signup and view all the answers

    What is formed after the reaction of the carbocation intermediate with water during hydration?

    <p>An oxonium ion</p> Signup and view all the answers

    In the acid-catalyzed hydration process, what happens in the final step?

    <p>Proton transfer to regenerate the acid catalyst</p> Signup and view all the answers

    Why can acid-catalyzed hydration not readily produce 1° alcohols?

    <p>The reaction favors more stable carbocations for the intermediates.</p> Signup and view all the answers

    During the acid-catalyzed hydration, what does the positively charged carbon in a carbocation exhibit?

    <p>sp2 hybridization</p> Signup and view all the answers

    Which of the following statements about carbocations is incorrect?

    <p>3° carbocations are the least stable.</p> Signup and view all the answers

    What does a larger activation energy (Ea) imply about the rate of a reaction?

    <p>Very few molecular collisions will have sufficient energy for the transition state.</p> Signup and view all the answers

    Which characteristic defines an exothermic reaction according to energy diagrams?

    <p>The energy of the products is lower than that of the reactants.</p> Signup and view all the answers

    In an energy diagram, what does the transition state represent?

    <p>An unstable species at maximum energy.</p> Signup and view all the answers

    Which statement accurately describes the heat of reaction ($ riangle H$)?

    <p>It indicates the energy change from reactants to products.</p> Signup and view all the answers

    What role does a reaction intermediate play in an energy diagram for a two-step reaction?

    <p>It is the energy minimum between two transition states.</p> Signup and view all the answers

    What characteristic of a carbocation affects its role as a Lewis acid?

    <p>It bears a positive charge.</p> Signup and view all the answers

    In terms of stability, which statement about carbocations is accurate?

    <p>A 3° carbocation is more stable than a 2° carbocation.</p> Signup and view all the answers

    What effect does the electronegativity of a carbon atom with a positive charge exert?

    <p>It withdraws electrons through an inductive effect.</p> Signup and view all the answers

    What is the relationship between carbocation stability and charge delocalization?

    <p>Greater delocalization often enhances stability.</p> Signup and view all the answers

    Why are methyl and 1° carbocations rarely observed in reactions?

    <p>They are extremely unstable and difficult to form.</p> Signup and view all the answers

    In a polar covalent bond, what is the charge characteristic of the more electronegative atom?

    <p>Partial negative charge (d–)</p> Signup and view all the answers

    When drawing a Lewis structure, which statement about the arrangement of electrons is true?

    <p>Nonbonding electrons are shown as pairs of dots.</p> Signup and view all the answers

    Which statement correctly describes the bonding requirements for carbon in neutral molecules?

    <p>Carbon typically has 4 bonds and no unshared electrons.</p> Signup and view all the answers

    What indicates a formal charge is positive on an atom in a molecule?

    <p>When the atom has fewer electrons assigned than in its neutral state.</p> Signup and view all the answers

    In an electron density model of a polar covalent bond, what does red signify?

    <p>A region of high electron density.</p> Signup and view all the answers

    Which statement correctly describes a property of polar molecules?

    <p>They have a geometric arrangement that results in an uneven distribution of charge.</p> Signup and view all the answers

    What must be true for two Lewis structures to be considered acceptable contributing structures?

    <p>They must have the same total number of paired and unpaired electrons.</p> Signup and view all the answers

    Which of the following correctly describes the role of curved arrows in resonance structures?

    <p>They indicate the redistribution of valence electrons.</p> Signup and view all the answers

    In what way do resonance structures contribute to the understanding of a molecule's properties?

    <p>They emphasize the average characteristics of the molecule.</p> Signup and view all the answers

    Which of the following statements concerning nonpolar molecules is correct?

    <p>They have symmetrical charge distribution.</p> Signup and view all the answers

    Which scenario defines a realistic representation of resonance?

    <p>A molecule is a hybrid of its various contributing structures.</p> Signup and view all the answers

    What is the primary criterion for validating a resonance structure?

    <p>It must maintain the number of valence electrons.</p> Signup and view all the answers

    What is the predicted bond angle in a methane molecule according to VSEPR theory?

    <p>109.5°</p> Signup and view all the answers

    Which shape is predicted for a molecule with a trigonal planar distribution of electron density?

    <p>Trigonal planar</p> Signup and view all the answers

    In terms of polarity, how does a water molecule behave due to its geometry?

    <p>Polar due to its bent shape</p> Signup and view all the answers

    What characteristic defines carbon dioxide as a nonpolar molecule despite having polar covalent bonds?

    <p>The bond dipoles cancel each other out.</p> Signup and view all the answers

    Regarding resonance structures, which option correctly describes the nature of these structures?

    <p>Resonance structures represent different possible arrangements of electrons.</p> Signup and view all the answers

    In a molecule exhibiting tetrahedral geometry, what is the influence of lone pairs on bond angles?

    <p>Lone pairs decrease bond angles.</p> Signup and view all the answers

    What is a distinguishing feature of the bond angles in benzene, C6H6?

    <p>They are all 120°.</p> Signup and view all the answers

    Which of the following accurately describes the electron density in acetylene?

    <p>It has a linear electron density arrangement.</p> Signup and view all the answers

    How does VSEPR theory predict the shape of molecules formed from two single bonded atoms and a lone pair?

    <p>Bent</p> Signup and view all the answers

    How many valence electrons are present in the nitrous oxide molecule, N2O?

    <p>10</p> Signup and view all the answers

    Which statement accurately describes the shape of a p orbital?

    <p>It consists of two lobes arranged in a straight line.</p> Signup and view all the answers

    Which is a consequence of hybridization in atomic orbitals?

    <p>It results in bond angles of approximately 109.5°, 120°, and 180°.</p> Signup and view all the answers

    When drawing contributing structures for a resonance hybrid, what is an essential aspect to include?

    <p>Formal charges on atoms, if present.</p> Signup and view all the answers

    What occurs during the overlap of atomic orbitals to form a covalent bond?

    <p>Partial atomic orbitals of two atoms combine.</p> Signup and view all the answers

    In the context of hybrid atomic orbitals, what does sp3 hybridization involve?

    <p>One s orbital and three p orbitals.</p> Signup and view all the answers

    What validates a contributing structure as unacceptably resonant in a molecule like N2O?

    <p>The arrangement of atoms without formal charge consideration.</p> Signup and view all the answers

    What defines the size relationship between 1s, 2s, and 3s orbitals?

    <p>3s orbitals are the largest.</p> Signup and view all the answers

    How do sp2 hybrid orbitals differ from sp3 hybrid orbitals?

    <p>sp2 consists of one s and two p orbitals.</p> Signup and view all the answers

    What is the characteristic feature of resonance structures in a molecule?

    <p>They must all exhibit the same connectivity.</p> Signup and view all the answers

    What factor has the greatest impact on the relative acidity of an organic acid?

    <p>The electronegativity of the atom bonded to H in HA</p> Signup and view all the answers

    Why are carboxylic acids stronger acids than alcohols?

    <p>The anion from carboxylic acids is stabilized by resonance</p> Signup and view all the answers

    How does inductive polarization influence acidity?

    <p>It stabilizes the anion by pulling electron density away</p> Signup and view all the answers

    Which option is NOT a factor in determining the stability of an anion?

    <p>pKa of the corresponding acid</p> Signup and view all the answers

    In terms of resonance structures, what is the primary reason for the greater acidity of carboxylic acids compared to alcohols?

    <p>Carboxylic acids can form more resonance structures</p> Signup and view all the answers

    Which statement regarding the stability of anions is accurate?

    <p>Greater stability of the anion correlates with stronger acids</p> Signup and view all the answers

    Which aspect does NOT contribute to the charge delocalization in an anion?

    <p>The size of the central atom</p> Signup and view all the answers

    How does the inductive effect differ from resonance stabilization?

    <p>Resonance involves delocalization of pi electrons; inductive effect does not</p> Signup and view all the answers

    What combination of characteristics can make an acid less acidic?

    <p>High electronegativity, poor resonance stabilization</p> Signup and view all the answers

    Which property does NOT enhance the stability of an anion directly?

    <p>Higher molecular weight of the base</p> Signup and view all the answers

    Study Notes

    Introduction to Organic Chemistry Chapter 5: Reactions of Alkenes and Alkynes

    • The chapter focuses on reactions of alkenes and alkynes
    • Table 5.1 details characteristic reactions of alkenes.

    Reactions of Alkenes

    • Hydrohalogenation: Addition of HX (e.g., HCl, HBr, HI) across the double bond
    • Hydration: Addition of water (H₂O/H₂SO₄)
    • Bromination: Addition of X₂ (e.g., Cl₂, Br₂) across the double bond
    • Hydroboration: Addition of BH₃ followed by oxidation

    Energy Diagrams

    • Energy diagrams graph energy changes during a chemical reaction
    • Energy is plotted on the y-axis and reaction progress on the x-axis
    • Figure 5.1 illustrates a one-step exothermic reaction of C and A-B to give C-A and B
    • Transition state is a maximum on the energy diagram, representing an unstable species during a reaction
    • Activation energy (Ea) is the difference in energy between reactants and the transition state. It determines the reaction rate. A large Ea indicates a slow reaction.

    Energy Diagram (Continued)

    • Heat of reaction (ΔH) is the difference in energy between reactants and products, determining if the reaction is exothermic or endothermic -Exothermic: Products have lower energy than reactants; heat is released -Endothermic: Products have higher energy than reactants; heat is absorbed

    Energy Diagram (Continued 2)

    • Figure 5.2 displays a two-step exothermic reaction involving intermediate formation
    • The reaction has distinct transition states and an intermediate
    • The rate-determining step is the step with the highest activation energy barrier, that is the slowest step

    Mechanism Patterns

    • Pattern 1: Adding a Proton
    • Adds a proton across the pi bond of a C=C double bond to form a new C-H bond.
    • Typical of acid-catalyzed reactions
    • Pattern 2: Taking a Proton Away
    • Reverses the process of adding a proton, with focus on the compound that loses the proton
    • Pattern 3: Reaction of an Electrophile and a Nucleophile
    • Reaction of an electron-poor species (electrophile) and an electron-rich species (nucleophile) to form a new covalent bond
    • Electrophiles are electron-poor species(Lewis acid). Nucleophiles are electron-rich species (Lewis base).
    • Pattern 4: Rearrangement of a bond
    • Rearrangements occur when bonding electrons from a sigma bond move to an adjacent atom.
    • Form a new covalent bond.
    • Pattern 5: Breaking a bond to form a stable molecule/ion
    • A carbocation can be formed when a bond breaks
    • The breaking of the bond releases the leaving group (e.g. bromide ion)

    Electrophilic Additions to Alkenes

    • Addition of hydrogen halides (HCl, HBr, HI)
    • Addition of water (H₂O/H₂SO₄)
    • Addition of halogens (Cl₂, Br₂).
    • Regioselectivity, Markovnikov's rule in addition of HX to a double bond, H adds to the carbon with the greater number of hydrogen atoms.

    Regioselectivity

    • Markovnikov's rule is one example of regioselectivity

    Carbocation

    • Figure 5.3 depicts the structure of a tert-butyl cation
    • A carbocation is a species with a positively-charged carbon atom, attached to three other atoms
    • Carbocations are highly unstable, and are often intermediates in chemical reactions.
    • Stable carbocations are most commonly 3* carbocations.

    Carbocation (Continued)

    • Carbocation stability: 3° > 2° > 1° > methyl. -A 3º carbocation is more stable than a 2° carbocation; requires lower activation energy for formation

    -A 2° carbocation is more stable than a 1° carbocation and requires lower activation energy for formation

    -Methyl and 1° carbocations are highly unstable. -Inductive effect -delocalization of charge over nearby atoms in a molecule leads to greater stability.

    Carbocation Rearrangements

    • Carbocation rearrangements happen in organic reactions
    • 1,2-shifts: alkyl or hydrogen migrations with bonding electrons to an adjacent electron-deficient atom, leading to a more stable carbocation.

    Addition of H₂O to an Alkene

    • Acid-catalyzed hydration of an alkene gives a protonated intermediate (e.g. oxonium ion)
    • Hydronium ion adds to the less substituted carbon.

    Reduction of Alkenes

    • Reaction with H₂ in the presence of a metal catalyst
    • Catalytic reduction or catalytic hydrogenation converts alkenes to alkanes.
    • Syn addition: Both H atoms add to the same face of the double bond. Hydrogenation of alkenes often involves syn stereoselectivity

    Reduction of Alkynes

    • Treatment of alkyne with H2 in the presence of a transition metal catalyst
    • Two moles of H2 add to the triple bond converting it to alkane.

    Addition of Chlorine and Bromine

    • A reaction of alkenes with halogens, carried out with either the pure reagents or an inert solvent.
    • A stereoselective reaction wherein one stereoisomer is preferentially formed or destroyed, and one stereoisomer is given preference over other products.
    • Syn-addition happens at three membered ring, the transition metal catalyst.

    Additional Notes

    • Problems are provided in the slides for students practice.
    • Mechanisms are illustrated for various addition reactions of alkenes, alkynes and carbocations.

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    Description

    Explore the reactions of alkenes and alkynes in this comprehensive quiz based on Chapter 5 of Organic Chemistry. Understand the mechanisms behind hydrohalogenation, hydration, bromination, and hydroboration, as well as the significance of energy diagrams in chemical reactions.

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