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Notes on Structure, Formulas, Skeletal Drawings, and Functional Groups

Structure, Formulas, Skeletal Drawings, and Functional Groups

  • Structure vs condensed vs molecular formula

    • Structure: information is condensed into a single line with connectivity order represented. Example: CH3-CH2-OH (left to right shows which groups are attached to which atoms).
    • Condensed formula: center atom first, then what is attached to it, then move to the next center atom and what’s attached. The left-to-right convention helps avoid ambiguity (e.g., CH3-CH(OH)-CH3 vs CH3-CH2-CH2-OH). In some cases you might write CH3CH2CH2OH; in others you show the central carbon first to reflect connectivity.
    • Molecular formula: lists atoms in the molecule by type and count, but gives no information about connectivity. Example: C3H8O.

  • Constitutional isomers

    • For a given molecular formula, there can be multiple structures (connectivities). These are called constitutional isomers.
    • Example: C3H8O has three constitutional isomers:
    • 1-propanol (propan-1-ol): ext{CH}3 ext{CH}2 ext{CH}_2 ext{OH}
    • 2-propanol (propan-2-ol or isopropanol): ext{CH}3 ext{CH(OH)} ext{CH}3
    • Ethyl methyl ether (ethyl methyl ether, an ether): ext{CH}3 ext{OCH}2 ext{CH}_3
    • Point: molecular formula alone is not enough to specify which molecule is being referred to.

  • Condensed formula conventions (in practice)

    • When listing a center atom, start with the atom that is the functional hub, then list what is attached to it.
    • Example discussion: for the first carbon, why would you not write CH3C–… first? The instructor notes that central-atom-first conventions can reduce ambiguity about what is attached to the center atom.
    • In a condensed formula, hydrogens on carbons are assumed unless otherwise shown; hydrogens on heteroatoms must be shown explicitly.

  • Skeletal structures vs Lewis structures

    • Skeletal structures are simplified drawings that convey a lot of information with fewer lines.
    • Lewis structures show explicit lone pairs and bonding electrons; skeletal structures omit many of these for simplicity but still convey connectivity.
    • They can convey more information about reactivity and functional groups than a crude Lewis drawing in some cases.
    • Example comparison: a Lewis structure might show a double bond to O with explicit lone pairs; the corresponding skeletal structure would show a C=O line with fragment connections.

  • Rules and interpretation for skeletal drawings

    • Endpoints, bends, and junctions (intersections) denote carbons.
    • Hydrogens on carbons are usually implied; hydrogens on heteroatoms (non-C, non-H) must be shown explicitly.
    • Heteroatoms are any atoms other than H or C.
    • Lone pairs are usually assumed unless drawn otherwise.
    • For geometry, start to consider hybridization and arrangement around centers as you learn about shapes (e.g., trigonal planar around carbonyl carbons).
    • Don’t rely on overly long or linear drawings for multiple bonds; try to reflect near-accurate geometry where possible.
    • Example: a carbonyl (C=O) carbon should be drawn with trigonal planar geometry around the carbon (120° bond angles), not a deceptively linear depiction.
    • Rotations around sigma bonds allow conformational flexibility; different zigzag drawings can represent the same molecule because sigma bonds rotate freely.
    • Conformation vs identity: rotating around single bonds changes shape but not connectivity or constitution.

  • Practical example: reaction sketch in skeletal form

    • A reaction that converts a double bond to a single bond with hydrogen (hydrogenation) is often easier to see in skeletal drawings: the double bond disappears, the hydrogen adds, etc.
    • Skeletal drawings can make functional-group transformations more immediately apparent than full Lewis structures.

  • Functional groups as organizing principles

    • Functional groups are common structural fragments that largely determine chemical properties and reactivity.
    • Organic chemistry is organized around functional groups; starting from chapter 4 onward, most chapters are structured by functional group.
    • The idea is to learn the properties and reactions of each functional group so you can predict behavior of molecules containing them.

  • Hydrocarbon functional groups (four total in this section)

    • Alkane: saturated hydrocarbon; only single bonds (sigma bonds); no pi bonds.
    • Alkene: carbon–carbon double bond (C=C).
    • Alkyne: carbon–carbon triple bond (C≡C).
    • Arene: cyclic arrangement with conjugated double bonds (benzene-like). In this course, the example shown is benzene; it’s a distinct class with unique reactivity compared to simple alkenes. Note that arenes will be studied in more detail later; for now, benzene is the representative example.
    • Important note: although alkane is often listed with functional groups, it is not strictly treated as a functional group in the same sense as the others; it is more of a scaffold used to compare other groups.

  • Notation for hydrocarbon and heteroatom fragments

    • Z: shorthand for a heteroatom (non-H, non-C).
    • X: halogen.
    • R: alkyl group (a carbon chain). Often used to denote generic substituents.
    • Rx: alkyl halide (a carbon chain with a halogen on it).

  • Functional groups containing heteroatoms (covering carbon–heteroatom bonds)

    • Alkyl halide: Rx (R = alkyl chain; X = halogen). Examples: alkyl bromide, bromobutane. General idea: carbon chain with a halogen substituent.
    • Alcohol: ROH (an -OH attached to a carbon chain). Example: ethanol. Also seen as R–OH in simple representations.
    • Ether: ROR (two carbon chains connected by an oxygen). Example: diethyl ether (Et2O). Hydrogens on the oxygens are not shown; symmetry of R groups can vary (R and R′ may be different).
    • Thiol: SH (functional analogue of alcohol with sulfur). Example name family: mercaptans. Thiols end in -thiol; sulfur-containing analogs of alcohols.
    • Sulfide (thioether): RSR (two carbon chains bound to sulfur). Example: dimethyl sulfide (DMS) would be Me2S (not shown explicitly here but conceptually similar).
    • Amine: NR3 (nitrogen with three substituents). Amines can be primary (one R group and two H), secondary (two R groups and one H), or tertiary (three R groups, no hydrogens on N). Examples shown include various combinations of alkyl groups and hydrogens.
    • Notes: If asked to label functional groups, you can simply label as an amine (don’t need to specify primary/secondary/tertiary unless asked).

  • Carbonyl-containing functional groups (C=O core)

    • Carbonyl: C=O; many functional groups arise from this motif depending on what is attached to the carbonyl carbon.
    • General pattern: R–C(=O)–R′ (the substituents on either side of the carbonyl define the specific functional group).
    • Aldehydes: R–CHO (one side is hydrogen). Example: formaldehyde, acetaldehyde is CH3CHO.
    • Ketones: R–CO–R′ (two carbon substituents; carbonyl carbon bonded to two carbons).
    • Carboxylic acids: R–COOH (carbonyl carbon also bonded to an OH group). The combination C=O and OH on the same carbon defines a carboxyl group.
    • Esters: R–COOR′ (an acyl group connected to an OR′ group). Example: ethyl acetate: ext{CH}3 ext{COOCH}2 ext{CH}_3.
    • Note on labeling: When you have a carbonyl attached to both an OH and a carbonyl, that specific arrangement is a carboxylic acid; if you only have an OH without the carbonyl, it's an alcohol. Do not label a single structure as both a carboxylic acid and an alcohol.
    • Additional carbonyl-containing groups discussed or implied include amides (R–CO–NR′R′′) and nitro groups (-NO2) as part of broader functional-group coverage; nitro groups will appear later in the course.

  • Specific examples and study tips mentioned

    • Ethyl acetate: ext{CH}3 ext{COOCH}2 ext{CH}_3 (an ester; common solvent).
    • Methoxyethane or methyl ethyl ether (for the C3H8O example): ext{CH}3 ext{OCH}2 ext{CH}_3.
    • Benzene as an archetype for arenes (aromatic ring).
    • Ammonia (NH3) relates to amines in terms of nitrogen bonding.
    • The functional-group focus is reinforced via on-paper practice, worksheets on Canvas, and a referenced app (Kim Functional Group) for quick lookups.

  • Practical exam and study considerations discussed

    • After today, expect functional groups to appear on every exam (intended coverage). Some previous quizzes or pages in Wiley might show discrepancies; the instructor emphasizes consistency and practice.
    • Flashcard-worthy items include nitro groups and amide structures; consider using flashcards and the Kim Functional Group app to drill recognition.
    • When labeling, focus on the family (e.g., amine, ether, aldehyde, ketone, carboxylic acid, ester, nitro, amide) and then refine as needed (primary/secondary/tertiary for amines, etc.).

  • Quick summary of key definitions and conventions to memorize

    • Heteroatom: any atom other than H or C.
    • Z = heteroatom shorthand; X = halogen; R = alkyl group; Rx = alkyl halide.
    • Alkane: saturated hydrocarbon; only single bonds; no pi bonds; scaffold not always treated as a functional group on exams.
    • Alkene: C=C; alkyne: C≡C; arene: benzene-like ring with conjugated double bonds.
    • Carbonyl-containing families: aldehydes (R–CHO), ketones (R–CO–R′), carboxylic acids (R–COOH), esters (R–COOR′), amides (R–CONR′R″), nitro (–NO2).

  • Organizational note for exam prep

    • Use the functional-group table (also on Canvas) for quick reference.
    • Practice converting between structure types (Lewis vs skeletal vs condensed) for given formulas, especially for isomer identification.
    • Practice drawing accurate geometries where possible (e.g., trigonal planar around carbonyl carbon) and recognizing when rotation around single bonds renders different drawings of the same molecule.