CHEM 281 - MIDTERM #1 EXAM QUESTIONS WITH CORRECT ANSWERS LATEST UPDATE 2026

CHEM 281 - MIDTERM #1 EXAM QUESTIONS WITH CORRECT ANSWERS LATEST UPDATE 2026 Carbon can NEVER have more than _____ bonds. - Answers 4 What is the difference between single bond & double bond rotation? - Answers Single bonds = rotate freely, positioning of groups doesn't matter Double bonds = restricted rotation due to energy barrier required to break & re-form pi bond, so positioning of groups matters Which conformation (chair or boat) of cyclohexane is more stable and why? - Answers Chair Why is cyclohexane the most stable alkane ring? - Answers It is the only one that is completely free of ring strain, its groups are in a perfectly staggered conformation How do we place large groups on cyclohexane for it to be most stable? - Answers In the EQUATORIAL position (if possible) How do we distinguish cis/trans for cyclohexane? - Answers If there are two substituents on cyclohexane... - if they are both pointing up or both pointing down (regardless of axial vs equatorial), it is cis - if one is pointing up and one is pointing down (again regardless of axial vs equatorial), it is trans What is a chiral carbon and what does it lead to? - Answers sp³ hybridized carbon with 4 different groups around it Leads to enantiomers (non-super imposable mirror image isomers) Distinguish between polar protic and polar aprotic solvents and give examples of each. - Answers Protic - have a donatable/acidic hydrogen which usually means they have hydrogen-bonding (ex: water, ethanol) Aprotic - lack any acidic hydrogens (ex: diethyl ether, acetone) What are isomers? How are their properties related? - Answers Different compounds with the same molecular formula Usually have very different properties What are constitutional isomers? - Answers Same molecular formula but different actual bonding relationships between atoms Distinguish between cis and trans isomers and what type of molecule they are important to. Which is more stable? - Answers Important to alkenes since bond rotation is restricted Cis = large groups are on same side of double bond Trans = large groups are on opposite side of double bond Trans is more stable because there is less interaction between large groups What is the difference between elements in the first 2 rows of the table and elements in the 3rd row and down in terms of bonding? - Answers First 2: obey octet rule 3rd and down: have d orbitals available for bonding, so can expand their valence to form more bonds than usual What is formal charge? How does it relate to the charge on the whole molecule? - Answers FC = number of electrons around an atom in relation to how many it usually has Sum of FC's on all atoms in a molecule equals the charge on the molecule What are resonance structures? - Answers Different Lewis structures that represent the same molecule; they exist only on paper and in real life the molecule is a hybrid of all of them What is the relationship between resonance and stability? - Answers Resonance stabilizes a molecule by delocalizing the charge. The more resonance structures a molecule has, the more stable it will be The energy of a resonance hybrid is ______ than the energy of contributing structures. - Answers Lower What is the relationship between number of covalent bonds and stability? - Answers The more covalent bonds, the more stable Charge separation decreases stability, this is why formal charge structures contribute less What are hybrid orbitals? - Answers They are an after the fact "invention" made to account for the real-life shapes of molecules They mathematically combine an atom's individual atomic orbitals into "hybrids" (like mixing 1 s and 3 p orbitals into an sp³ hybrid orbital) in order to make orbitals of equivalent energy How do you determine the hybridization state of an atom? - Answers Count # of things around an atom - bonds count as one thing, regardless if they are double/single/triple - lone pairs count as one thing Number of things matches hybridization (2 = sp, 3 = sp²...) What are molecular orbitals? - Answers Joint orbitals formed by bonding atoms from their individual atomic orbitals Energy of electrons in bonding MO's is _______ than in their individual AO's. - Answers Lower Distinguish between sigma and pi bonds and how they are formed. - Answers Sigma bonds = all single bonds, they are formed from end-to-end overlap of orbitals Pi bonds = form the 2nd and 3rd bonds in double/triple bonds, they are formed from side-to-side overlap of orbitals Distinguish between the Aufbua principle, the Pauli exclusion principle, and Hund's rule. - Answers Aufbau - losest energy orbitals are filled first Pauli exclusion - only two electrons may occupy a given orbital and their spins must be paired Hund's rule - degenerate orbitals receive one unpaired electron first before electrons are paired How do we figure out molecular shape? - Answers Must determine hybridization/how many things are around your atom, then distinguish between the number that are lone pairs and the number that are bonds List the molecular shapes. - Answers 2 things - 0 LP - linear 3 things - 0 LP - trigonal planar 3 things - 1 LP - bent 4 things - 0 LP - tetrahedral 4 things - 1 LP - trigonal pyramidal 4 things - 2 LP - bent 5 things - 0 LP - trigonal bipyramidal 5 things - 1 LP - see-saw 5 things - 2 LP - T-shape 5 things - 3 LP - linear 6 things - 0 LP - octahedral 6 things - 1 LP - square pyramidal 6 things - 1 LP - square planar Distinguish between alkanes, alkenes, and alkynes and where they are found/used. - Answers Alkanes - all single bonds, found in natural gas and petroleum Alkenes - at least 1 DB, found in industrial use as building blocks and in natural compounds (5 carbons) Alkynes - at least 1 TB, also found in industrial use or drugs to "push" groups out (linear shape) What special features are required for an alkane to be called an aromatic? - Answers 1. ring-shaped 2. flat or planar 3. conjugated system (alternating DB and SB) Why is benzene toxic? Why is toluene a better choice? - Answers Flat, planar structure allows it to slip between rungs of DNA and cause cancer, also hard to metabolize because so non-polar and unreactive Toluene has extra methyl group that adds some reactivity so it is easier to metabolize Why does benzene have resonance stabilization and what does this mean for its structure? - Answers Double bonds can move around Leads to "cloud" of delocalized pi electrons above and below ring What's more acidic, phenol or methanol? - Answers Phenol, because removing its H leaves it with lots of resonance whereas methanol will have none Define electronegativity. - Answers Intrinsic ability of an atom to attract shared electrons to itself How does size of electronegativity difference relate to polarity of bond? - Answers Larger EN difference = more polar bond What is the general trend of electronegativity on the periodic table? - Answers Across a group = more EN Up a column = more EN Explain the criteria that must be met for a molecule to be considered polar. - Answers 1. must contain polar bonds (EN difference between atoms) 2. shape must cooperate - dipole moments cannot cancel How does polarity relate to cis/trans isomers? - Answers Cis isomers are polar because they will have a net dipole directed towards the side the groups are on Trans isomers are non-polar because their dipoles cancel due to opposite positioning of groups What do alkane groups do to a molecule? - Answers Push electron density in (because of C-H bond polarity directed at C) What is the major difference between benzyl and phenyl groups? - Answers Phenyl - just regular benzene Benzyl - benzene with CH₂ attached The CH₂ has ACIDIC hydrogens because their removal will lead to resonance, so they are much more reactive What do halogens do to a molecule? - Answers Pull electron density away, because they are very EN Ethers are what kind of solvent? - Answers Polar aprotic Amines make good _______, particularly ____ amines. Why? - Answers bases, secondary They have a lone pair to donate which makes them good bases; secondary in particular because they still have room to access the LP (not too bulky) but also have enough alkyl groups to push e⁻ density in How do you distinguish between primary, secondary, etc amines? - Answers By number of carbon groups that N itself is attached to What is pyridine? - Answers Aromatic ring (benzene style) with an N in place of a carbon Which is more reactive, aldehyde or ketone and why? - Answers Aldehyde - it is more positive, because less alkyl pushing in, and it is easier to access the C since it's on the end What do we call the C and H's next to a carbonyl group and what is special about them? - Answers Alpha C's and H's The H's are acidic because removal of them will lead to resonance Explain how a carboxylic acid leads to an ester, a halogen, an amide, and an anhydride. - Answers Ester - reacts with OH Halogen - reacts with halogen acid Amide - reacts with amine Anhydride - two acids react together What are ion-ion forces? - Answers Forces between positive and negative ions that hold it together in its crystalline state, VERY strong Most ionic organic compounds do what before boiling? - Answers Decompose - the ion-ion forces are so strong that they need a very high temperature to break, and this temperature is usually not reached before it decomposes the structure itself Name and define the 3 van der Waals forces. - Answers 1. dipole-dipole forces: these are between polar molecules, δ+ area of one is attracted to δ+ of other 2. hydrogen bonds: these are a special type of dipole-dipole that are particularly strong and exist when H is bonded to O, N, or F 3. dispersion forces: exist between all molecules, temporary dipoles caused by random motion of electrons that can further induce dipoles and attraction in neighbouring molecules Distinguish between an H-bond acceptor and an H-bond donor. - Answers You can be an acceptor without being a donor, but a donor is always also an acceptor. Acceptor: don't have H-bonds within themselves necessarily, but has lone pairs on EN atoms that attract the H's from the donor molecule Donor: the ones with the H bonds, making them have a very δ+ H that is attracted to lone pairs on either the same type of molecule or a different acceptor What is one of the most important consequences of H-bonding? - Answers It allows water to be a liquid instead of a gas at room temperature - it should be a gas based on size and mass! What are the 2 factors determining how strong dispersion forces are between molecules? - Answers 1. size of atoms - larger atoms are more polarizable because their electrons are more loosely held, cloud vs point charge (ex: Cl vs F) 2. entire molecule surface area - larger surface area means more room for attraction (ex: pentane vs neopentane) How do intermolecular forces relate to boiling point? - Answers More IM forces = higher BP because more energy has to be put in to separate the molecules into the gas phase What are the general rules for solubility? - Answers Like dissolves like Non-polar solutes dissolve in non-polar solvent Polar and ionic solutes dissolve in polar solvents Why can't non-polar and polar liquids mix? - Answers The IM forces of attraction between molecules of the same type greatly exceed the attraction between molecules of different types, so they stay with their own type Distinguish between hydrophobic and hydrophilic. - Answers Hydrophobic - incompatible with water Hydrophilic - compatible with water What is the structure of a soap molecule and how does it work? - Answers Long "fatty chain" (C-H) with a polar hydrophilic group at the end It works because the C-H part dissolves in grease and the polar part dissolves in water, and they wash away together Also the long fatty chain disrupts bacterial cell membranes, so all soaps are antimicrobial What is the general rule for water solubility in terms of # of carbons? - Answers 1-3 carbons water-soluble 4-5 borderline 6+ water-insoluble What are the 4 basic categories of organic reaction mechanisms and some defining features? - Answers Substitution - usually with SB or aromatic compounds, involves one group replacing another Elimination - one small molecule leaves a larger one, typically increasing bond order Addition - two molecules combine into one, typically drops bond order Rearrangement - no change in number or type of atoms, just a reorganization (reactant and product are constitutional isomers)