Organic Chemistry 1 · Structure and electron accounting
Bonding and formal charge: practice problems
44 problems, each with its worked solution. Open one to read every step, or filter by tag.
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- Assigning a departing electron pair
- Making a bond from a lone pair
- Why borane comes dissolved in THF
- Hydroxide captures carbon dioxide
- Acetone bonds to boron trifluoride: carbonyl activation
- Ammonia adds to carbon dioxide: carbamate zwitterion
- Heterolysis of benzhydryl chloride: diphenylmethyl cation
- Bromide bonds to iron(III) bromide: tetrabromoferrate
- Hydride adds to borane: forming the borohydride ion
Draw the mechanism 16
- Assigning a departing electron pairRepresent heterolysis with one pair arrow and conserve total charge.
- Making a bond from a lone pairBuild a Lewis acid–base adduct while updating both formal charges.
- Why borane comes dissolved in THFForm the borane–THF complex from an ether oxygen lone pair and assign the oxonium and borate formal charges it creates.
- Hydroxide captures carbon dioxideAdd hydroxide to carbon dioxide by breaking exactly one C=O π bond, then delocalize bicarbonate’s charge over its two equivalent oxygens.
- Acetone bonds to boron trifluoride: carbonyl activationDraw the carbonyl oxygen of acetone donating to boron trifluoride and explain how the adduct activates the carbonyl carbon.
- Ammonia adds to carbon dioxide: carbamate zwitterionDraw ammonia adding to carbon dioxide, assign the formal charges of the carbamate zwitterion and give the new hybridization of carbon.
- Heterolysis of benzhydryl chloride: diphenylmethyl cationDraw the heterolysis of benzhydryl chloride and explain why the diphenylmethyl cation is unusually stable.
- Bromide bonds to iron(III) bromide: tetrabromoferrateDraw bromide donating a lone pair to iron(III) bromide and assign the formal charge of the tetrabromoferrate ion.
- Hydride adds to borane: forming the borohydride ionDraw hydride donating its electron pair to borane and predict the shape and formal charge of the borohydride ion.
- Methyllithium meets waterDraw the C–Li bond pair of methyllithium taking a proton from water and explain why its carbon behaves like a carbanion with a −1 formal charge.
- Methylamine takes a proton from hydrogen chlorideDraw the proton transfer from hydrogen chloride to methylamine and assign the formal charge on nitrogen in methylammonium.
- Ammonia bonds to boron trifluorideDraw the arrow by which ammonia donates its lone pair to boron trifluoride and assign formal charges in the adduct.
- Heterolysis makes a carbocationDraw the heterolysis of the C–Br bond of tert-butyl bromide and describe the bonding and geometry of the carbocation formed.
- Methanol captures the tert-butyl cation: an oxonium ionDraw methanol capturing the tert-butyl cation and assign the formal charge on oxygen in the oxonium ion formed.
- Trimethylamine bonds to borane: an amine–borane adductDraw trimethylamine donating its lone pair to borane and describe how the hybridization of boron changes in the adduct.
- Water gains a protonDraw water taking a proton from HCl and assign the formal charge on the hydronium oxygen.
Predict the product 3
- Bond angle around the carbonyl carbonPredict a bond angle from the electron-domain geometry of the central atom.
- Which structure has a positively charged nitrogen?Locate formal charges by counting bonds and lone pairs on nitrogen.
- Geometry of the methyl cation versus the methyl anionPredict the hybridization and shape of a carbocation and a carbanion from their electron domains.
Classify 3
- An odd electron is not a pairClassify an odd-electron species by electron count.
- Hybridisation of the atoms in acetonitrileAssign sp, sp2 or sp3 hybridisation from the number of electron domains around each atom.
- Which molecules break the octet rule: BF3, SF6, NH3, CH4Count the electrons around a central atom to classify incomplete octets, expanded octets and normal octets.
Compare 5
- Two meanings of chargeSeparate partial bond polarization from formal electron ownership.
- Rank carbon–carbon bond lengthsRank carbon–carbon bond lengths by bond order.
- Rank C–F, C–O, C–N and C–H bond polarityRank bonds to carbon by polarity using the electronegativity difference between the two atoms.
- Rank C–H bond strengths by carbon hybridizationRank C–H bond dissociation energies using the s character of carbon and the stability of the radical formed.
- Rank molecular dipole momentsRank molecules by dipole moment by combining bond polarity with molecular geometry.
Calculate 5
- A nitrogen electron ledgerCalculate formal charge from bonding and nonbonding electrons.
- Formal charge on nitrogen in nitromethaneCalculate the formal charge on a nitrogen that forms four bonds in a nitro group.
- Count lone pairs in methyl acetateCount all lone pairs in a neutral molecule by completing octets on heteroatoms.
- Count σ bonds in acrylonitrileCount σ bonds in a molecule containing double and triple bonds, including bonds to hydrogen.
- Hydrogens in a skeletal structureCount implicit hydrogens in a skeletal structure to write its molecular formula.
Find the error 2
- An electron-deficient carbonDiagnose why a carbocation can be valid without an octet.
- What is wrong with this carbon dioxide Lewis structure?Diagnose an incorrect Lewis structure by checking octets and formal charges, then repair it with multiple bonds.
Interpret the evidence 3
- Reading a triple bondDistinguish σ and π components in a multiple bond.
- Hybridization of the central carbon in alleneAssign hybridization in a cumulated diene and relate the orthogonal π bonds to its three-dimensional shape.
- Skeletal structure of (CH3)2CHCH2OH (isobutanol)Translate a branched condensed formula into its skeletal structure by reading each group in order.
Work backward 2
- Reconstructing cyanideInfer nonbonding electrons from total charge and octets.
- Ozone Lewis structure from its formal chargesReconstruct a Lewis structure from given formal charges and the valence electron count.
Plan a synthesis 2
- A Lewis-structure construction planChoose a construction satisfying total electron count and two octets.
- Plan the Lewis structure of the azide ion, N3−Order the steps for building a Lewis structure of a polyatomic ion and minimizing its formal charges.
Explain why 3
- Geometry at an iminium nitrogenRelate a multiple bond and σ directions to local geometry.
- Why the nitrogen of N,N-dimethylacetamide is planarExplain how resonance delocalization of a lone pair changes the hybridization of an amide nitrogen.
- Which orbital of bromomethane accepts hydroxide in SN2?Identify the acceptor orbital in an SN2 reaction and connect it to backside attack and bond breaking.
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