What's The Difference Between H3C And CH3? - Physics Forums

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What's the difference between H3C and CH3?

• Thread starter Thread starter tony873004
• Start date Start date Feb 3, 2012
• Tags Tags Difference

Click For Summary

Discussion Overview

The discussion revolves around the notation differences between H3C and CH3 as representations of the methyl group in molecular structures, specifically in the context of caffeine and theobromine. Participants also explore the bond angles associated with these groups, addressing both theoretical and visual representations.

Discussion Character

• Technical explanation
• Conceptual clarification
• Debate/contested

Main Points Raised

• Some participants note that H3C and CH3 are notational variations for the same methyl group, with no inherent difference in meaning.
• One participant suggests that the notation may visually indicate the bonding orientation of carbon and hydrogen atoms.
• Another participant expresses a preference for writing methyl groups in a specific format, indicating that there is no universally accepted convention for these notations.
• There is a claim that the ideal bond angle for H-C-H in methyl is 109.5º, with variations possible depending on the surrounding molecular structure.
• One participant mentions that a 120º bond angle is characteristic of molecules with three electron pairs, contrasting this with the tetrahedral geometry of CH3, which has four electron pairs.

Areas of Agreement / Disagreement

Participants generally agree that H3C and CH3 represent the same group, but there is no consensus on the preferred notation or the implications of bond angles, as differing views on these aspects are presented.

Contextual Notes

Participants reference bond angles and molecular geometry, indicating that these may vary based on the specific molecular context, but do not resolve the discrepancies in angle measurements mentioned.

tony873004 Science Advisor Gold Member Messages 1,753 Reaction score 143 When looking at the "shiny ball" models of a caffeine molecule, I notice that it terminates in 3 places with a carbon atom bonded to 3 hydrogens. But diagrams from different sources list two of them as CH3 and one of them as H3C. What's the difference? Also, what are the spacings in H3C / CH3? It looks like each bond is 120 degrees from the other bonds. But somewhere I recall hearing 105 degrees. I'm probably mixing this up with something else. Thanks in advance! Chemistry news on Phys.org

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DaveC426913 Gold Member 2025 Award Messages 24,378 Reaction score 8,600 I noticed that too, when I was looking at theobromine (almost identical to caffeine). I assumed it was just to visually indicate that the N (or C in your case) is the one with the bond, not the H's, but I don't really know. Ygggdrasil Science Advisor Insights Author Gold Member Messages 3,753 Reaction score 4,198 There's no difference. Sometimes it's more convenient to write the three hydrogens on the left side of the carbon and sometimes its more convenient to write the hydrogens on the right side of the carbon. Borek Mentor Messages 29,183 Reaction score 4,619 I tend to write them in the form H3C-CH3 - that is, leaving carbons on the bond side (as opposed to CH3H3C - which suggests to me carbons being bonded through hydrogen; sometimes that would make sense though, like in HBH2BH - not that I would ever write diborane this way). But that's just my approach, there is no generally accepted rule/convention here. Just like some people write -CO2H and others write -COOH for carboxyl group. juanrga Messages 476 Reaction score 0

tony873004 said: When looking at the "shiny ball" models of a caffeine molecule, I notice that it terminates in 3 places with a carbon atom bonded to 3 hydrogens. But diagrams from different sources list two of them as CH3 and one of them as H3C. What's the difference? Also, what are the spacings in H3C / CH3? It looks like each bond is 120 degrees from the other bonds. But somewhere I recall hearing 105 degrees. I'm probably mixing this up with something else. Thanks in advance!

As others said you both CH3 and H3C are notations for the same methyl group -CH3. Regarding your other question the ideal bond angle H-C-H in methyl is 109.5º. Small variations may be waited in function of the nature of the fragment (R) to which is linked this group R-CH3 If you do a 2D projection of the group it must look as 120º, but this is fictitious. Last edited: Feb 4, 2012 AlchemistK Messages 157 Reaction score 0 A 120º bond angle is formed when there are just three pair of electron (lone pairs or bond pairs) around an atom. One example being BF3. -CH3 on the other hand has 4 electron pairs around it, and hence forms a 3D geometric figure, a tetrahedral, to ensure minimum repulsion between electrons. This structure has an angle of 109º28' (But can change slightly in case of unlike bonds) tony873004 Science Advisor Gold Member Messages 1,753 Reaction score 143 Thanks for the replies!

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